(feat)(Stackmob):Prevent relationships from being submitted with PUT and POST requests

This was causing Stackmob to say that the relationship was of invalid type because it would see [Object object] instead of the relationship_id String that it was expecting
(feat)(Stackmob):Allow valid arrays, disallow arrays of objects

Of course this will need to be modified to allow for multiple relationship updating.
(refactor)(Stackmob):Refactor stackmob-spec.js to not repeat so many mock objects
(chore):Merge with README.md changes