Recommend using the bcrypt library instead of py-bcrypt

* py-bcrypt has not been updated in some time
* py-bcrypt does not support Python3
* py3k-bcrypt, a port of py-bcrypt to python3 is not compatible
  with Django
* bcrypt is supported on all versions of Python that Django
  supports
Merge pull request #1062 from dstufft/switch-bcrypt-recommendations

Recommend using the bcrypt library instead of py-bcrypt