[WIP] Rewrite the Swift support to work with iOS 7

There's still some build system and documentation stuff to do, but all tests pass and I was able to submit a Swift app targeting iOS 7 and using Realm for review with this.

Happens to fix #898 as a side effect.
The point of making the instance in RLMProperty was to avoid making it if it wasn't needed... but then I never actually made it do that. Probably not worth it anyway since it turned out that string properties need it as well.