Support python 3

- various unicode/bytes test fixes

 - resolve an (undocumented?) change in how ctypes converts unicode
   strings to char string by always using the default filesystem
   encoding.

Test Plan:
adam@gaba:~/github/python-magic$ python test.py
.....
----------------------------------------------------------------------
Ran 5 tests in 0.010s

OK
adam@gaba:~/github/python-magic$ python3 test.py
.....
----------------------------------------------------------------------
Ran 5 tests in 0.016s

OK
adam@gaba:~/github/python-magic$ ~/opt/python3.3/bin/python3 test.py
.. snip ...
----------------------------------------------------------------------
Ran 5 tests in 0.025s