Replace 'Caret Range' jQuery plugin by 'TextRange'

Fixes #4 text duplication issue when caret is or selection starts at
position 0.

The snippet insertion code is also simpler.
Minify jquery-textrange.js
Add Readme file for jQuery plugins
Update copyright year
Capitalize 'Create' button label
Set maximum requirement to MantisCore < 1.3

The CSS changes introduced in 1.3 cause the plugin's config pages to
generate errors.
Fix behavior of Select all check box in snippet lists

The change event now uses 'prop' function instead of 'attr'.
Bump jQuery requirement to 1.6

This is required for use of prop() function