Simplify _get_cipher
Add property callback to CryptoEngine

This way we can easily change "public" or "private"
access to callback
Remove set_callback
Disable set_callback
Change how callback is controlled

Remove that from the __init__ method
More cleanup of CryptoEngine
More cleanup of the really bad CryptoEngine constructor
Yay, CryptoEngine in a bit more testtable form
Make Crypto.auth python3 compatible
More python3 compatibility

* disable __str__ of NewTag, it causes
a bug in CryptoEngine
More Python3 compatibility for crypto.py

* Also, update a doc-string
Remove all calls to _callback.<method>

Instead call the methods as if they where methods of CryptoEngine
itself
Multiple python3 fixes for crypto.py
Multiple fixes for pep8 and python3
Fix for python3
Multiple python3 compatibility fixes