Fix for <Leader>x and other minor issues

Fixed a bug in <Leader>x (todo#PrependDate) introduced last commit.
Fixed case sensitivity bug in todo#ToggleMarkAsDone() and todo#UnMarkAsDone()
Fixed errors being reported for repeat#set if vim-repeat plugin is not installed.
Fixed modeline in .vim files to work (only works on first/last 5 lines of file), and made the modelines consistent across all files.
New unit tests for todo#ToggleMarkAsDone()
Minor fixes for README.md and man page
Fix anchoring of RegExp in todo#ToggleMarkAsDone
Fixed a bug in overdue highlighting that resulted in the current date being matched on round dates like the 20th and 30th.
Added more unit tests for overdue date highlighting.
Corrected RegExp anchoring on today date highlighting.
Added a few bash scripts to make running unit tests easier and more reliable, including testing with ignorecase user preference set.

doc/todo.txt

@@ -31,9 +31,9 @@ Table of Contents                                          *TodoTxt-Contents* ~
 ===============================================================================
 1. Release notes                                       *TodoTxt-ReleaseNotes* ~
 
-V0.8.1 Incorporates yet antoher Fretep work : highlithing for tasks due today.
+V0.8.1 Incorporates yet another Fretep work : highlighting for tasks due today.
 
-v0.8 Incorporates Fretep's work on overdue dates (PR#13 and PR#16) witch removes
+v0.8 Incorporates Fretep's work on overdue dates (PR#13 and PR#16) which removes
 python dependency, allow to control the cursor position after a sort by todo
 (see |TodoTxt-Sort| and/or issue #15) and fixes bug when sorting a file
 containing only lines with due:date (issue #14).
@@ -66,7 +66,7 @@ by David Beniamine.
     http://todotxt.com
 
     "The todo.txt format is a simple set of
-    rules(https://github.com/ginatrapani/todo.txt-cli/wiki/The-Todo.txt-Format)
+    rules(https://github.com/todotxt/todotxt/)
     that make todo.txt both human and machine-readable. The format supports
     priorities, creation and completion dates, projects and contexts. That's
     all you need to be productive. See an example Todo.txt file":
@@ -83,7 +83,7 @@ by David Beniamine.
 2.2 Why this fork ?                                              *TodoTxt-Fork*
 
     This plugin is a fork of freitass todo.txt-vim
-    (https://github.com/freitass/todo.txt-vim). It add severals cool
+    (https://github.com/freitass/todo.txt-vim). It add several cool
     functionalities including:
 
     + Hierarchical sort: |TodoTxt-Sort| and |TodoTxt-HierarchicalSort|.
@@ -126,7 +126,7 @@ by David Beniamine.
 ===============================================================================
 3. TodoTxt Files                                              *TodoTxt-Files* ~
 
-This plugin provides a Flexible file naming for todo.txt, all the following
+This plugin provides a flexible file naming for todo.txt, all the following
 names are recognized as todo:
 >
     YYYY-MM-[Tt]odo.txt
@@ -210,7 +210,7 @@ For more information on the available flags see |:sort|
 ===============================================================================
 6. Mappings                                                *TodoTxt-Mappings* ~
 
-By default todo-txt.vim set all the mappings secribed in this section. To
+By default todo-txt.vim set all the mappings described in this section. To
 prevent this behavior, add the following line to your vimrc
 >
    let g:Todo_txt_do_not_map=1
@@ -224,8 +224,6 @@ prevent this behavior, add the following line to your vimrc
 
     `<LocalLeader>s@` : Sort the file on @Contexts
 
-    `<LocalLeader>sd` : Sort the file on due dates
-
     `<LocalLeader>sc` : Sort the file by context then by priority
 
     `<LocalLeader>scp` : Sort the file by context, project then by priority
@@ -234,8 +232,9 @@ prevent this behavior, add the following line to your vimrc
 
     `<LocalLeader>spc` : Sort the file by project, context then by priority
 
-    `<leader>-sd` : Sort the file by due-date. Entries with a due date appears
-    sorted by at the beginning of the file, the rest of the file is not modified.
+    `<LocalLeader>sd` : Sort the file on due dates. Entries with a due date appear
+    sorted by at the beginning of the file, completed tasks are moved to the bottom and
+    the rest of the file is not modified.
 
 When you sort by due dates, at the end of the sort, your cursor will be placed
 at the top of the file. This behavior can be set with the following global