Improve style of list of playlists in playlists tab.

- Adds a background color to the active playlist.
- Moves the song count badge to the right of the playlist and includes
  the word "songs" to incdicate what we're counting.

![image](https://cloud.githubusercontent.com/assets/17613/4007227/7e808b0a-29c0-11e4-9576-480fe47b3ff6.png)