Moving product groups and scopes into an extension

[cmar]

Product groups and scopes have been moved into an extension https://github.com/spree/spree_product_groups

[cmar]

@BDQ This pull request should be ready now. The calculator and product_spec tests are passing with all_tests.

Thanks to @LBRapid for helping to get the tests passing

[runlevel5]

@cmar: please let me know once this is pulled so I could add in few note for the release note as well modifying details about product groups on spree-guides

[BDQ]

Merged.

@joneslee85