Use destructors to free client and server state

[behackett]

This change obsoletes the functions authGSSClientClean and authGSSServerClean. The client and server state are now destroyed when the PyCapsule / PyCObject storing them are reclaimed. The functions remain to avoid breaking backward compatibility, but just return AUTH_GSS_COMPLETE.

Resolves #48.

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[jborean93]

Have checked this locally and can confirm every is good.

[behackett]

@dreness - this one resolves #48