Adds support for `requests`.

Requests wasn't working because it uses urllib3, which uses a custom version of HTTPConnection. I introduced support by mocking and patching this class. Also added tweaks to maintain compatibility with some weird things urllib3 does.

Closes issue #7.
Yup, thanks for catching. I need to test without requests installed. Or do you think it makes sense to make it a required dependency? It's pretty lightweight and would simplify the code.