Let 'ys' theme use hg repo info too

This small change basically lets the `ys` theme use hg repo info, in addition to the git info it already supports. We don't use vcs_info because all we want to know is if the repo is clean or dirty.

Original Git implementation:
![screen shot 2014-08-13 at 11 59 08 pm](https://cloud.githubusercontent.com/assets/251281/3906412/b615e472-22f2-11e4-91c1-1bf6ee2df644.png)
![screen shot 2014-08-13 at 11 59 20 pm](https://cloud.githubusercontent.com/assets/251281/3906413/b61726ac-22f2-11e4-8f5c-2ad053e0a231.png)

New HG implementation:
![screen shot 2014-08-13 at 11 58 39 pm](https://cloud.githubusercontent.com/assets/251281/3906415/beffbf86-22f2-11e4-8d2b-7dc12595948e.png)
![screen shot 2014-08-13 at 11 58 49 pm](https://cloud.githubusercontent.com/assets/251281/3906416/bf67ce28-22f2-11e4-9599-dd5ed87fed64.png)


I haven't done much in the shell, so please let me know if there are any issues!