Added the ability for the dev to choose which edge(s) that the ChatHead will snap

Now the developer can choose which edge(s) that the ChatHead will snap: CHSnappingEdgeBoth, CHSnappingEdgeRight and CHSnappingEdgeLeft.
To set which edge you want to use, simply call
```objective-c
_draggingCoordinator.snappingEdge = CHSnappingEdgeBoth;
```
_draggingCoordinator being the CHDraggingCoordinator.

Thanks!