Made a few CSS changes for WordPress 3.8+

Noted in the HTML5 audio player option that Firefox 26.0+ now supports mp3 playback. We are aware that Firefox 21.0 supported mp3 playback but it was limited to particular Windows versions.

Cleaned up some HTML in the admin area.

git-svn-id: https://plugins.svn.wordpress.org/powerpress/trunk@832035 b8457f37-d9ea-0310-8a92-e5e31aec5664