Lightweight Core Data Migration of RestKit

ref: RAYWENDERLICH's Core Data Migration tutorial

Core Data’s Lightweight migration is a great feature, use it whenever possible.
RestKit is sitting on top of Core Data, and RestKit make Core Data Migration simpler than original.

The few things we have to do is:

• Select .xcdatamodeld file and choose Editor > Add Model Version
• If there were Entity or Attribute renamed, refering to the old data from renaming ID field

Give a glance at my code, there's a bit differnt from Pure Core Data because of RKManagedObjectStore.