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 wanted it outside of otter.util, because the way I see it, otter.util is for things that otherwise aren't otter-specific, and could as well be third-party library code. I originally named it "auth_http" because the first feature I added was integration with otter.auth. maybe "otter.service_http", "otter.http", "otter.rax_http"?