Add shipping category to New Product Form

@RohanM @oeoeaio: Couple of questions on this PR
1) I suspect aesthetically (and logically) this will conflict with @Matt-Yorkley PR to add tax categories. Happy to redo to make it more pretty once his has been merged.

2) Do I need specs for this change? My gut feeling was no, because this is an optional category already supported in spree. Let me know if I'm wrong. 

Thanks!