Added keyboard support

Added support for checking a checkbox by using the keyboard return key (Char Code 13).
Updated outline colour

Changed outline colour as advised to a more pleasing one.
Added sync to original checkbox

When using keyboard on our styled checkboxes, sync state back to original checkbox.
Added constants and changed bind to call

Better readability for constants.
Changed to space for selection

Using space instead of enter for selection as suggested.
Merge pull request #331 from jasonmayes/gh-pages

Fixes for keyboard support and outline colour