Adds ability to specify a layer for selection

I have three layers and I wanted to show a selection cursor on top of the second layer.  Because the grid selection's superview was always the first layer, if the second layer was present on a tile, it would simply not show (hidden under second layer).  This allows you, in your grid settings when defining layers, to specify a layer as the one on which the selection will appear.  My example:

```javascript
exports = {
	tileWidth: 82,
	tileHeight: 41,
	layers: [{}, {
            selectionLayer: true
		}, {
			dynamicViews: 30
		}
	]

};
```

The selection layer defaults to 0, as before.  As always I'm open to any ideas about syntax or variable names.