Create and update of relationship linkages

Description

This is a bit of milestone release!
Thanks to @BenjaminHorn via #73 we now support create/update/delete of relationship linkages during the create and update of a resource. With this feature addition, we now provide enough functionality to be useful for an ember js developer.

The only feature we need to land to make life better for ember developers is side loading data which is currently in the works.

It's also worth noting that we now have 100 passing integration tests so I feel like we have a fairly solid solution and am happy to change our 'alpha' label to a 'beta' label. We welcome feedback and especially bug reports so we can fix anything ahead of a 1.0 release.