Switch from python-gnupg to gnupg, update calls

* "gnupg" is a fork of python-gnupg which seems more actively maintained and
  more responsive to bug reports than python-gnupg
* Updates to the latest version of gnupg and updates gnupg calls to
  preserve semantics where the API changed
Fix nose tests to work with updated gnupg API
Merge pull request #13 from edx/jrbl/bugfix_gpg

Switch from python-gnupg to gnupg, update calls