Create Feed model

We'll be persisting Feed data using Core Data.
Therefore, we need a data model file to define
Feed entity and its attributes.
Setup single context Core Data stack
Create feed data manager

All data operations related to a feed should go
through this data manager instead of accessing
NSManagedObjectContext directly from CoreDataStack.
Merge pull request #1 from pawanpoudel/features/setup_data_layer

Setup Data Layer