Added method objectsInRange: to RLMCollection

Allows users to get an NSArray of instantiated accessors. I think the naming here makes sense as it pairs nicely with ```objectAtIndex:``` which is our other accessor creation method.

@tgoyne @jpsim 
I think so. We need to specify that we aren't returning a crazy custom NSArray subclass that behaves very differently from normal ones, which is all that really says.

s/from/in/, though.