Support for oauth with RSA-SHA1 signing

This is #1042 + tests and docs, and also a change to use the `private_key` OAuth parameter for RSA-SHA1 signing instead of the `consumer_secret` parameter which is used for HMAC-SHA1 signing.  Per [the RFC](https://tools.ietf.org/html/rfc5849#section-3.4.2) this is correct.

@FredKSchott @mikeal or anyone else: review please?  Also, here are a couple of test scripts against http://oauthbin.com/ that give me confidence that this works correctly:  https://gist.github.com/nylen/639ef899d9964cb2957c