Rename GroupPricing to Pricing

* rename GroupPricing result class, and test to Pricing. issue [#116 ]
* rename Product many_to_many relationship with Pricing from group_pricings to prices.
* update MANIFEST to reflect class/test name change.
Since we're using Candy and this is an API change we don't need to declare 'table' anymore as we can let Candy autotable handle it.