Integrated GnuPG via the gnugp-python module

The KeybasePublicKey class now creates a gnupg.GPG class instance within itself. This is the tool that will do the real, heavy lifting for us when it comes to encrypting and verifying signatures.

The class setups up a completely private keyring for the GPG class that contains one and only one key: the Keybase key associated with the KeybasePublicKey class instance. When the class instance is destroyed so is the temporary keyring. In this way the user's default keyring is never polluted.

TODO:

* Still have to write the KeybasePublicKey.encrypt() and .sign() methods
Some minor refactoring

Moved some things around to make it all a little cleaner. Added some static methods because those are cool, right?
Successfully connected gnupg-python to the KeybasePublicKey class.

I had originally wanted to use PyCrypto's RSA implementation here but I could not get the Keybase public keys to play nice with the RSA key implementation in PyCrypto. The fallback to gnupg isn't so bad -- KeybasePublicKey still adds a whole bunch of syntactic sugar to the gnupg.GPG() class that'll make it easier to work with.

Next up: encryption and signature verification!