Add managed object mapper for CoreData object mapping
Add CoreData section to README
Don't create empty managed objects

For managed objects if there is an empty representation it would create
an empty managed object, which would make validations fail. Now if the
representation is empty no object is created.
Convert NSNull values to nil
Reorganize sources, add CoreData example project and tests
Fix problem with Pods config for the test target
Merge pull request #4 from aleph7/core_data

Support for managed objects