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

```

To be merged right after the final 0.12 release, in case this breaks something,
learn that fact in master rather then have users discover it when upgrading.
 Please mention me if this merge creates issues.
