Lowering Simperium Memory Footprint

- Adding SPDictionaryStorage class:
  Based on NSCache + CoreData, serves as a memory efficient cache, with disk persistence.

- Updated SPChangeProcessor to work with the new SPDictionaryStorage

- Updated SPChangeProcessor to split changes in several partitions, to lower the amount of RAM required when the client app has a large number of "offline changes".

This pull request addresses Simperium Issue #132: https://github.com/Simperium/simperium-ios/issues/132

Demo App used to test this pull request: https://github.com/jleandroperez/simperium-memory
Validated with 1500 offline objects, 10kb each, on an iPad Mini.