Use the python unittest framework for tests of utils.py
Clean up PBKDF2 class

We currently include the PBKDF2 implementation from the PyCrypto author.
This patch removes some of the stuff we do not use and need. In the
future we should move to the PBKDF2 implementation that is nowadays included
in PyCrypto to clean up the code even more.
Add license header to tests and fix formating