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
`headers = kwargs.pop('headers', {})`. Also this would raise `KeyError` if header is not passed in original function which in reality wouldn't happen since `get_request_func` is adding headers. But still this should be handled.