`mix deps`: Display refs pointing to the checked out commit for each dep

Hi guys. What do you think about this? The implementation is very dirty, I just wanted to hack something quickly to get your feedback on the feature.

Before:
```
λ mix deps    
* gproc 0.2.16-19-g6f7f6c3 (git://github.com/esl/gproc.git)
  locked at 6f7f6c3
  ok
* ranch 0.9.0 (git://github.com/extend/ranch.git)
  locked at 5df1f22
  ok
* jazz 0.0.2 (git://github.com/meh/jazz.git)
  locked at 6733246
  ok
* cowlib 0.4.0 (git://github.com/extend/cowlib.git)
  locked at 63298e8
  ok
* cowboy 0.9.0 (git://github.com/extend/cowboy.git)
  locked at 5a25c7f
  ok
```

After (notice added refs after each commit hash):
```
λ mix deps    
* gproc 0.2.16-19-g6f7f6c3 (git://github.com/esl/gproc.git)
  locked at 6f7f6c3 (origin/master, master)
  ok
* ranch 0.9.0 (git://github.com/extend/ranch.git)
  locked at 5df1f22 (tag: 0.9.0, origin/master)
  ok
* jazz 0.0.2 (git://github.com/meh/jazz.git)
  locked at 6733246 (origin/master, master)
  ok
* cowlib 0.4.0 (git://github.com/extend/cowlib.git)
  locked at 63298e8 (tag: 0.4.0, origin/master, master)
  ok
* cowboy 0.9.0 (git://github.com/extend/cowboy.git)
  locked at 5a25c7f (origin/master)
  ok
```

We could also add colouring like git does:
![screen shot 2013-11-15 at 00 20 49](https://f.cloud.github.com/assets/207748/1545750/11ee5a84-4d7b-11e3-9b16-fdbbe81d3477.png)