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
I'm not sure I understand. Putting it here makes it possible to determine the reauth_codes and success_codes at every call site, which is what you want, right? Putting it in get_request_func means you have to know ahead of time. maybe I don't understand the use case you're suggesting