BLD: pandas should generate PEP 440 compliant dev versions
Doing an inplace install with recent versions of pip (e.g., with `pip install -e .`) currently produces this warning:
`UserWarning: The version specified ('0.15.2-224-geadfd92') is an invalid version, this may not work as expected with newer versions of setuptools, pip, and PyPI. Please see PEP 440 for more details.`

We should update setup.py to produce PEP 440 compliant versions such as `'0.15.2+224.geadfd92'` instead. For an example of how to convert between, see https://github.com/warner/python-versioneer/pull/45#issuecomment-74085795
