Additions to Return Authorizations API

I wrote three additions (add variant, mark as received, mark as canceled) to the Return Authorizations API because it did not offer all of the functionality that the admin interface had.  I needed it so that an outside system can manage these for my Spree implementation. I tried to design them in line with similar methods on the Shipments API.

I also wrote these for the 2-0-stable branch because I'm not using 2.1 in production yet.  If you guys like these additions, pull them in. I can write them into the Spree docs if this pull request get accepted.