deprecate use of gnutls

Although gnutls (or, eventually, pypolarssl!) is a long term goal, right now we have many libs in the client depending on openssl. gnutls support is adding packaging overhead (because some bugs, and because of the subtleties of ctypes CDDL support in pyinstaller).

So, moving the few functions that make use of gnutls to use PyOpenSSL instead.

(from redmine: created on 2013-01-24, closed on 2013-01-24)