Add support for two-way SSL

This commit allows specifying a key and cert file along with the username and password to support two-way SSL.  The server cert is still ignored, but now the provided client key is sent.

This only works if Python was compiled with SSL support.  Although it relies on httplib (which was removed in a previous commit), it should work with Python 2.4, but I didn't test it on anything other than 2.7.