Isolate core better from mappings

[supermarin]

We should probably make ObjectiveRecord/Core, sub spec in case someone wants no magic and to stay simple.
This also includes a bit of architectural changes in NSManagedObject+ActiveRecord.m, to check if mappings are present.

[conradwt]

@mneorr This sounds like a great idea. For example, if fields change within my application from v1 to v2, then I would simply perform a migration to remove those unused fields from the application.