Add tmuxline extension

As discussed, I've used `add_theme_func` to hook up to `:AirlineTheme`. Nice :)

However, I'm not sure if this extension is ready to be merged into airline just yet:
- tmuxline.vim still lacks a `doc/tmuxline.vim` file (will write it within the next few days)
- tmuxline.vim probably has some bugs lurking around (I've tested it on latest vim + latest tmux + latest iterm2 only)

Let me know if you have some ideas on how tmux integration can be improved.

Showcase:
![tmuxline-showcase](https://f.cloud.github.com/assets/1532071/1559276/4c28fbac-4fc7-11e3-90ef-7e833d980f98.gif)

Refs #345