require registration of resource types

[csantero]

This PR paves the way for polymorphism (#73) and heterogeneous collection support by requiring the user to pre-register all resource types. This will be a major breaking change as existing apps will fail until they add their necessary registrations.

I changed the language of GetJsonKeyForType to GetResouceTypeNameForType because the spec no longer wants primary data to be keyed by type, but rather by the word data. "Resource type name" seemed a more apt description of what we are determining.

I also added a virtual method on ModelManager called CalculateResourceTypeNameForType which will allow sub-classes to override the default strategy for determining the resource type name.

@SphtKr do you have any concerns?

[SphtKr]

Yeah, not thrilled about that requirement, but don't see a good way around it right now. Json.Net is able to skirt any registration requirement because it essentially requires you to tell it the type it's deserializing before it starts...with the type key in the resource we won't have that luxury.

[SphtKr]

LGTM. Couple of thoughts for the future, it might make sense to go ahead and put any registered types through the wringer of all the other caching methods, to build up the caches at startup (but that's internal and we can do it later). Also, I'd favor adding a method to add everything in a namespace, and probably some way to add everything in a DbContext, for convenience. The latter will have to go in JSONAPI.EntityFramework somewhere of course...I imagine we should probably have an EF-specific subclass of ModelManager in there at some point, and such a method would make sense there.