added method to get both unblurred and blurred snapshots in an array

@smbarne I was wanting to get both a snapshot and blurred snapshot of a view without having to get a context or call drawViewHierarchyInRect twice in a row. In my case, this involves wanting to create a blur fade-in effect for a mapview where I can also hide the mapview once it has been snapshotted. 
Not sure I'm crazy about the fact that this returns an array that is intended to be used as a tuple. Especially since one or both values can be `NSNull`, which tends to be a constant source of bugs in ObjC (and not documented in the comments, BTW). You might consider a tiny struct-like class to hold the result values so it's more explicit and nil-safe.