Removed gssapi.client and related files

gssapi/client.py was really more SASL-related,
and belongs in a separate project.
Use Kerberos Test Harness for Unit Tests

Previously, it was necessary to have a fully functioning
Kerberos setup with a 'host' service for the unit tests to
run properly.  Now they instead use a heavily modified version
of the MIT Kerberos test harness (k5test.py), so only Kerberos
binaries and libararies are needed, not a working installation.

This has the added benefit of bein self-contained.
Made *SecContext check for cred=None case

Previously, if cred=None, we would still attempt to
pass cred.raw_cred in `initSecContext` and `acceptSecContext`.
In Python 2.7, this would succeed for some reason.
It fails (like it probably should) in Python 3.3.
The issue is now fixed.
Extra None-handling logic

Several 'not None' statements were added to parameters,
as well as None-handling logic in compareNames