Expose current service/method in the Context

[danjacques]

Add EndpointInfo() method to pull the ServiceInfo and MethodInfo of the
current endpoint out of the Context.

Defer generation of the context to after the service/method have been
identified. This prevents unnecessary setup operations (including
AppEngine binding), until after the request is verified to be
referencing a valid endpoint.

This is a focused landing of the embedding portion of #125 built on top of #126 as requested.

[riannucci]

Wondering if there's any intent to pull this?

It would be super-handy for centralizing more advanced authorization logic (like fine-grained group membership/ACL checks). In fact, if there was a way to get the validated/decoded request body too, that would be fantastic. I'd like to have a static mapping of endpoint->authorization_functions and be able to ensure that it is applied consistently to all endpoint handlers within a given service. Right now I have to copypasta boilerplate (with duplicated method names to do map lookups) at the top of each endpoint handler... always nervous that we'll add/modify a handler and the boilerplate will have a bug (point at the wrong method, handle the error improperly, omit the boilerplate entirely, etc.)