Pass pluginPath to MediaElement.js, fixing playback when ME.js is served through load-scripts.php.

Merges [24940] to the 3.6 branch.

props ocean90, wonderboymusic, azaozz.
fixes #24902.



git-svn-id: https://core.svn.wordpress.org/branches/3.6@24941 1a063a9b-81f0-0310-95a4-ce76da25c4cd