Theme Customizer: Use native postMessage for wp.customize.Messenger. see #19910.

* Removes use of jquery.postmessage.js
* Fixes bug where Opera would attempt to use the hash transport and redirect to the dashboard.
* Fixes bug where multiple postMessage connections could not coexist in a single frame.



git-svn-id: https://core.svn.wordpress.org/trunk@20517 1a063a9b-81f0-0310-95a4-ce76da25c4cd