[WIP] Refactor CollectionView Datasource/Delegate handling

- [x] Move UICollectionViewDelegate implementation from MBContactPicker to ContactCollectionView
- [x] Move UICollectionViewFlowlayoutDelegate implementation from MBContactPicker to ContactCollectionView
- [x] Move UICollectionViewDataSource implementation from MBContactPicker to ContactCollectionView
- [ ] Validate UIResponder behavior after above changes are made
- [ ] Per @MattCBowman's comment, remove the ```entryCell``` property and instead use a private ```NSString*``` property to persist the text that should be in the ContactCollectionViewEntryCell
- [x] Move UITextFieldDelegate to the collection view as it's just concerned with backspaces, leave UIControlEventEditingChanged delegate intact