Service-bound HTTP requests

This introduces an abstraction for "service-bound request functions", which allow you to perform HTTP requests without knowledge of the tenant, name of the endpoint in the config file, region, or authentication.

It's based on the pure-http-refinements branch, so to see only the changes this branch makes, see here: https://github.com/rackerlabs/otter/compare/pure-http-refinements...service-bound-http
Do we ever deploy anything without CachingAuthenticator? If not, then I think it's fine, mostly because I really want to get this stuff working so I can unblock everyone else :) I'm happy to look into it later, though.