Switched hash functions from PyCrypto to hashlib.

There's a few advantages to this:

1) It's probably faster, OpenSSL, which typically backs hashlib, receives far
   more attention for optimizaitons than PyCrypto.
2) It's the first step to supporting PyPy, where PyCrypto doesn't run.
3) Convention dictates to use the stdlib if there's not some reason not to.