Use hashlib instead of md5, even for python 2

Pystan already requires python 2.7, which comes with hashlib.
Actually, hashlib comes with all versions of python since 2.5.

I see a deprecation warning everytime I import pystan (using
python 2.7) because python tells me md5 is deprecated.
Merge pull request #28 from shoyer/use-hashlib

Use hashlib instead of md5, even for python 2