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.
This is meant to check whether the client is registered and valid. While not a bad idea for this type of endpoint it is slightly more than just checking the signature. Was this what you intended? If so I'm happy to merge.