Fix building on older Pythons

[behackett]

This change fixes building on Python 2.4 (the oldest I have available) through 2.6 and 3.1. It also fixes a few compiler warnings.

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[behackett]

My main goal here was to re-enable support for Python 2.6 so projects like PyMongo and requests-kerberos can switch back to ccs-pykerberos from various forks. Supporting Python 2.4 and 2.5 just happens to be the same change.

[jborean93]

Have tested this manually without any errors against the following versions;

• 2.6.9
• 2.7.13
• 3.3.6
• 3.4.6
• 3.5.3
• 3.6.2

[behackett]

@dreness - any comment on this?

[dreness]

I'm going to do the thing where I merge something right before going offline, in this case for two weeks. But, I don't think this is too risky, plus the tests are passing :)