IMPROVEMENTS
- moved ServiceType from otter.convergence to otter.constants
- made get_request_func's return value accept a service type as the
  first argument, because functions making requests should be able to
  determine what service they want to talk to.
- added an add_headers wrapper function to otter.util.pure_http,
  like add_effectful_headers but without the effect. @manishtomar,
  you were right :)
- there's now no longer a double-authentication when using a bound
  service.