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.
This was @tgoyne 's addition, but it seems to be the same api as NSSortDescriptor - this method is needed precisely because ```RLMSortDescriptor``` is immutable and you can't simply set the ascending flag.