Export available widgets to JS and add widget select
Populate available widgets select with Backbone collection
Refactor building of setting args into helper method
Register non-active widget settings as well
Only apply jshint to staged JS files
Add ajax groundwork for handing newly added widgets
Move setupDeleteControl from sidebar_widgets into widget_form

Closes #41
Add support for single (non-multi) widgets
Prepare widget control for instantiation
Use filter_input() and do widget option update sanity check
Export default transport with each availavle widget
Add new widget setting and control to section

Setting is not yet hooked up with preview
Filter preview so incoming widget additions will be recognized
Refactor logic for deleting widgets to rely on model
Make widget addition, deletion, and reordering all model-driven

Allow old widgets to be resurrected, e.g. via Settings Revisions
Ensure widgets added in customizer persist
Fold standalone updatePreview method into sortable update event
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