Feature/py3k

Have mostly adapted the jose package to work with python 3 (in addition to python 2).  
Getting tests running with multiple versions of python has necessitated rearanging some of the files but I believe that I have managed to avoid breaking the current api.

There is one remaining problem which is that I can't figure out what format the key for hmac algorithms should be in and therefore whether, and where, it should be encoded as bytes or unicode.  Will have another look at fixing it now.  This problem does not affect python 2 however.

I do have to say that the excellent test suite has made this really quite easy.  Thank you.