BLD: Use 'git describe' to generate the version string for dev versions 

closes https://github.com/pydata/pandas/issues/4046
```
λ sudo python ./setup.py develop
...
Installed /home/user1/src/pandas
Processing dependencies for pandas==v0.12.0rc1-48-g9442eac
Searching for numpy==1.7.1
Best match: numpy 1.7.1
Adding numpy 1.7.1 to easy-install.pth file

λ python -c 'import pandas; print pandas.version.version'
v0.12.0rc1-48-g9442eac

```