Create Generic OAuth1 Endpoint

In certain cases a provider may wish to verify the signature of an
oauth request without doing anything more with it. Learning Tools
Interoperability (LTI), for example, uses "0-legged OAuth" for it's
signature verification process.

http://www.imsglobal.org/lti/
http://andyfmiller.com/2013/02/10/does-lti-use-oauth/

This adds a new ``GenericEndpoint`` that implements only the client
validation and signature verification, and leaves off the other
parts that would need to be validated.
It is what I intended. For LTI, which is driving this endpoint, it would (almost) always to make sense to check the client key. Obviously, we can't have a secret without a key, so the key needs to be registered.

The operations seems common enough, even for a 0-legged case, that I think that it would be strange if it were not checked the same way as for the other endpoints that use the client key.