installation: mercurial addition

In case mercurial is not installed on the system, `$ pip install -r requirements.txt` fails with:

```
Obtaining editdist from hg+https://code.google.com/p/py-editdist/#egg=editdist (from -r requirements.txt (line 2))
  Cloning hg https://code.google.com/p/py-editdist/ to /home/jlavik/envs/nextstructure/src/editdist
Cleaning up...
Cannot find command 'hg'
```
See discussion at #1814