Create Signature Only 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 ``SignatureOnlyEndpoint`` that implements only the
client validation and signature verification, and leaves off the other
parts that would need to be validated.