Use 3.4's ssl.create_default_context when available

Related #472 
This should probaly do something on Python 2.7 yea? (Maybe 3.1 too? I forget if 3.1 supports ciphers to ``wrap_socket`` or not). Also do we want it to silently do nothing on 2.6, or raise an error if you attempt to use it on 2.6 (since you can't actually set ciphers on 2.6)