Upgrade our TLS support by using pyOpenSSL-backed requests.

requests will automatically use pyOpenSSL if it, ndg-httpsclient, and
pyasn1 are installed. pyOpenSSL is currently the best TLS implementation
for Python, and gives us features above the standard librarly ssl module
like TLS SNI support.

You'll need to re-run `pip install -r requirements.txt` and note that
you'll need the libffi headers to compile pyOpenSSL for platforms like
Linux that don't have binary dists available from PyPI (such as Linux).
(Install package `libffi-dev` on Debian/Ubuntu.)

You can verify that yours is working properly by running:

    py.test -s tests/general/network/test_sni.py

Thanks to Hynek Schlawack's PyCon 2014 talk on SSL/TLS for this. :)