Override MP6-introduced color from white back to black

Some of the text in widget controls was becoming white because MP6
changes the color scheme of accordions from dark on light to light on
dark. But the forms are still light, so the text inside needs to
remain dark.

Fixes #33
Only apply jshint to staged JS files
Make old single widget available again after removal
Reorganize sidebar_widgets model setup
Fix failure for new widget additions to trigger preview refresh
Prevent old single widgets from not obeying instance override
Move option transacrions into separate class
Fix up handling of RSS widget which fails to template itself

Remove dead code
Eliminate retrieve_widgets() by managing wp_inactive_widgets directly
Elimiante closures for PHP 5.2 compat
Fix typo which broke widget additions
Fix dragging between sidebars
Keep form_control sidebar_id accurate when dragged to another sidebar
Fix trashing widgets with corrected jQuery.contains params
Remove RSS Feed widget fixups

The retrieve_widgets() function may have been causing the problem.
Add nonce checks for prepreview setup
Merge pull request #3 from x-team/issue-3-addition

Add support for adding widgets
Merge branch 'develop' into issue-25
Update unit tests for latest changes in issue-3-addition branch