Multiple Contexts Support

Hello.

I'm facing multiple contexts issue for a long time. I need to have an persistent Store and an InMemory Store in my app. I like ObjectiveRecord, But the useInMemoryStore isn't the solution. So I make up my mind to do something about it.

I added the manageObjectContexts dictionary and related methods into ObjectiveRecord. So I can Add / Remove Contexts into ObjectiveRecord; Fetch / Find objects from multiple contexts with an identifier; Move / Copy objects between contexts.

I'm not used to do testing on my code. So these changes are not tested yet. I make this merge to the master branch so I can know what you guys think about these changes.

Cheers.