Primary key support for int and string columns

Also support for upsert for objects which have primary keys.

Missing:
- proper core support for indexing (both strings and ints)
- migration support and verification
@eni9889 this commit should have fixed the issues you were having: https://github.com/realm/realm-cocoa/commit/7d333839b5ed2e452fcb690b34bb5501f4e19a60

Please let us know if this is not the case.