Use postMessage as default transport for core widgets

Allow transport to be filtered
Listen for postMessage updates to widget settings
Invoke Ajax request to render widget for injection
Replace old widget with newly-rendered widget in DOM without refresh
Render widgets on current REQUEST_URI not admin-ajax

The Jetpack Widget Visibility adds filters based on whether is_admin(), so these work now.
Likewise, widgets may show/hide based on whether they are rendered on a certain page,
so this will also work now.
Allow removed widget to be restored with postMessage
Allow new widgets to be added to sidebars with postMessage
Sort widgets with postMessage
Enable postMessage updates based on opt-in availability

Add 'widget-customizer' theme support feature
Fix addition of newly-added uninitialized widgets in preview

Correct logic for obtaining list of postMessage-eligible widgets

Fixes #61
Capture preview and previewer, allow preview to trigger refresh
Trigger refresh of preview when last widget removed from sidebar
Remove unused JS function setting_id_to_widget_id()
Show spinner when updating widgets via postMessage
Fix widget sidebar reassignment postMessage enabled
Add live preview theme support for bundled themes

Especially for twentythirteen which uses Masonry to lay out widgets in a sidebar.

Other themes can hook into wp.customize.bind( 'sidebar-updated', function ( sidebar_id ) { ... } )
Fix unnecessary preview refresh when adding a widget
Simplify conditions for twentythirteen theme support JS
Opt-in to postMessage Ajax previews in twentyfourteen
Simplify widget live-previewable opt-in filter to use boolean