Pursuant to PEP466, this is a backport of Python 3.4's hashlib.pbkdf2_hmac.

Of note in this patch:

* None of the utilities for testing both a python and a C implementation simultaneously were present, so this only tests whichever implementation is available.
* Due to a variety of API changes and missing APIs, the code is not an exact copy-paste, tough luck :-)
* I haven't done docs yet.
* It currently accepts unicode values because the ``y*`` format from Python3 doesn't have any parallel in Python2. I'm not sure whether consistency with the rest of the 2-verse is more important than consistency with a sane way to treat data / the 3-verse.