Fix selector for applying drag-over style
Add alpha state for WYSIWYG editing

Some improvements still needed:
 * Formatting toolbar buttons need icons
 * Buttons may need to have an active state when selecting that formatted text
 * Improve round-trip between textarea and contenteditble
 * Localized strings for prompts
 * Prepopulating link and image URL in prompt when updating value

Tested so far in Chrome and Firefox. Needs testing in Safari and MSIE.

For Automattic/liveblog#97