make "bind_service" consistent with other request decorators, thus "add_bind_service"
add a json_response=True parameter.
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.
better docs for get_request_func
Address manish's review comments
Merge branch 'service-bound-http' into rcv3-requests

Conflicts:
	otter/convergence.py
Add RCv3 constant
Remove now-unused ValueConstant