Updated tmhOAuth.php to support RSA-SHA1 Signing.

I chose this library to handle OAuth authentication for a Xero Integration project, which worked great until using it for 'Private' applications (http://developer.xero.com/documentation/getting-started/private-applications/). Xero uses HMAC-SHA1 for public/partner applications and the RSA-SHA1 signature method for Private applications, which tmhOAuth doesn't support.

As Xero usage is growing rapidly, it'd be nice to incorporate these features into the main library.