Added support for stdlib's pbkdf2

This change should default the pbkdf2 implementation to the stdlib's whenever it is available. This is currently only Python 3.4 but should be available for Python [2.7.8](https://docs.python.org/2.7/library/hashlib.html#hashlib.pbkdf2_hmac) whenever it is released.

Nothing *should* break although I am not 100% confident of this because this bit of code doesn't appear to be covered by tests.