Split RLMArray into RLMArray + RLMResults

Splits RLMArray into separate apis.

The relevant apis should now be updated along with inline docs. All osx tests currently pass. We will still need to update the rest of the docs on the website separately.
Immutability is semi-relevant because it's still just one line to create an reversed descriptor:

```objc
[oldDescriptor reversedSortDescriptor]
```
vs
```objc
[RLMSortDescriptor sortDescriptorWithProperty:oldDescriptor.property ascending:!oldDescriptor.ascending]
```

Ok, so the first one is a lot clearer and less verbose. Let's keep it in.