Switch away from 'requests' and standardize SCMTool HTTP usage.

The 'requests' module, while nice on the outside, does not meet our
needs today. It doesn't support Python 2.4 or 2.5 (which RB 1.6.x does
support), and it's unclear what support will be going forward.

In reality, the main reason we're using it in 1.6.x at all is because we
needed to pre-populate the Authorization header, which requests will do
automatically. However, this isn't anything special. We can just add
that header ourselves and keep urllib2, which is fine because our needs
there are very basic.

In order to prevent code duplication, I've created a SCMClient class
that Git's and Mercurial's clients inherit from. It provides at the
moment one utility function, get_file_http, which wraps the urllib2
logic. If we switch back at some point to requests or another module, we
need only update this function. We probably won't need to, though.

In the end, both classes are simplified, we don't need this dependency,
we keep our Python support, and pre-populated authorizations work.

Tested this with a couple different Git and Mercurial repos, with and without
HTTP Basic Auth.

In particular, I made sure this still worked with CodebaseHQ's Git repository.

I also confirmed with Wireshark that the first request had the Authorization
header.