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.
github3.py does it in the docstrings often. This also applies to internal references, e.g.,

```rst
:class:`User <github3.users.User>`\ s
```