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
It seems like there's a bit of a disconnect between the module name and the docstring/contents. I'd say this description is accurate, and the module should probably be renamed, or merged with an already existing module.