Add support for connect timeouts

Modifies the timeout interface to also accept a tuple (connect, read) which
would be used to set individual connect and read timeouts for Requests. Adds
Advanced documentation explaining the interface and providing guidance for
timeout values.
So, this is sort of silly. My initial idea was to add the Timeout class from urllib3 to the docs and add an "internals" docs page. Since we're not using that interface I decided not to do it, ended up just reverting this change.