[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.
Perhaps we should instead have a different initializer for RLMProperty, one used for swift classes and one for objc. then we don't have to pass in cls unnecessarily.

Also, we should only be creating a single instance for each swift object type, so this method should have an argument called ```objectInstance``` which takes an id.