Jetpaack: Infinite Scroll: ensure compatibility with Core's `[audio]` and `[video]` shortcodes.

Building upon r815060 and r815118, Core's `MediaElement.js` scripts must be loaded before `document.ready`, otherwise the scripts aren't properly initialized and IS dies due to JS errors.

Also, realizing that some scripts were being enqueued at `wp_head` and others at `wp_enqueue_scripts`, I've consolidated the enqueueing within a method tied to the proper action.


git-svn-id: https://plugins.svn.wordpress.org/jetpack/trunk@815610 b8457f37-d9ea-0310-8a92-e5e31aec5664