Support older Python 2.7 versions

[clach04]

Python 2.7.2 and 2.7.3 struct.unpack() fail with:

error: unpack requires a string argument of length 4

If key is passed in as a bytearray parameter, this is fine in later 2.7.x versions and Python 3.x. Casting as bytes() works for both Python 2.7.x and 3.x.

http://stackoverflow.com/questions/20612587/struct-unpack-with-bytearrays has more information

[clach04]

Can this be merged?