Add RSA-SHA1 support

This builds on a fork from https://github.com/wraithgar/node-oauth.  All I did was to basically add a test that ensures creating a signature with RSA-SHA1 works and that this signature can then be verified with a matching public key.

(See comments in https://github.com/ciaranj/node-oauth/pull/43).

I also verified that this works against a working OAuth service provider using RSA-SHA1 (Atlassian JIRA).