Adds `.mejs-offscreen` styles for handling screen-reader text. Fixes: https://github.com/justintadlock/theme-mediaelement/issues/5
Time slider fixes for WP 4.1.
Horizontal volume control fixes for WP 4.1. See: https://github.com/justintadlock/theme-mediaelement/issues/6

Props @jayj
Time span margin adjustments for slightly better alignment.
Time rail padding/margin fix so that the time doesn't overflow its container.
Version bump to 20141221.