Added *_path_rfc6570 and *_url_rfc6570 helpers like Rails ones.

Also fixed missing opts argument for *_rfc6570 helpers.

These changes add convenient way to make path-only RFC6570 templates. 
RFC6570 is very convenient to use on both client and server sides, but path-only templates are more convenient to build "web-client-only" version of API (independence from hostname).