Use 3.4's ssl.create_default_context when available

Related #472 
On 2.7.0-2.7.8 it should pass the given ciphers into the ``wrap_socket`` call.

On 2.6 and 3.1 it should either silently do nothing (which is what it does now) or it should raise an error saying that you cannot set ciphers on this Python version. I don't have a strong opinion one way or another which it should be.