Twenty Thirteen: Add styles for mediaelement players

[obenland]

As the title says.

Add support for the audio shortcode and make it look great.

Proposed patch still needs to be run by Joen and a ton of testing.

23821.diff definitely looks better.

Before: ​http://cl.ly/image/2J283M2X3w0y

After: ​http://cl.ly/image/431K3m1R111R

Added styles for the audio and video player based on Joen's feedback: styled-players.png​.

Also flipped the audio icon for RTL: flipped-icon.png​

In 23781:

Twenty Thirteen: style up the new mediaelement audio and video players. Props obenland, fixes #23821.

@obenland Can you take another look at this, for the video player? Black background, or the same dark color the audio player uses.

In 23835:

Twenty Thirteen: style native mediaelement video player with a black background color. Props obenland, closes #23821.