Disable SSLv2 and SSLv3 from being used where possible

This will hopefully mitigate POODLE and all attacks related to SSLv2
and SSLv3 for versions of Python that allow us to disable this via
SSLContext options. Currently that excludes Python 2.6 and Python 2.7.
Python 2.7.9 should include the ability to disable these.

cc @Lukasa @dstufft @alex

Related issues: #471, #487