Enables installation of Network and ObjectMapping without Core Data (via CocoaPods)

This pull request adds a bunch of conditional compilation to allow for the installation of the Network and ObjectMapping sub specs via CocoaPods without requiring the inclusion of Core Data. Previously any attempt to use such a configuration would result in build and linker failures.

I am not totally in love with the amount of conditional compilation required to make this go, but it also does not feel good that the sub spec modules cannot build and function independently.