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

[ghost]

closes #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

[ghost]

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.

[ghost]

Please mention me if this merge creates issues.