Implement Query Product Variant [sphere.io] according to documentation. ...

Implement Query Product Variant [sphere.io] according to documentation. Fixes #733. Also added JSCS Syntax checker task for Grunt
We just decided that with should emit data with the ``rebound`` event. It might be a simple string describing why rebound is happening or in case of an error the error itself. This data will be sent to sentry. 

In this case I suggest to send a string like "Failed to find a product variant by id 1234"