Clean Up Setup.py Before Publish

`setup.py` will now automatically detect support for Cython and
compile with Cython when available.  Otherwise, it will try to
compile with the C files.  `MANIFEST.in` now keeps the C files
along with the Cython files, so that running `setup.py sdist`
will keep the C files in the generated source distribution.

Finally, the "author" of the project has been changed to
"The Python GSSAPI Team".  The license has been updated to
reflect this.  The actual license itself has been changed into
a more streamlined ISC license.
Don't compile from cython when installing sdist

It is reccomended not to have setup.py build from the Cython files
when users are installing from an sdist, even if they have Cython
installed.  Thus, the sdist command now creates a temorary file
included in the sdist file that instructs setup.py not to use Cython.
If setup.py detects this file, it will not even attempt to look for
Cython; it simply compiles from C.