Add bounds support

It appears Leaflet always had bounds support on TileLayer, but [it wasn't documented until recently](https://github.com/Leaflet/Leaflet/commit/f2ab783dbf765f91fd865ea0b4f7d103974f8368). I just noticed it and decided to quickly add support, based on [MapBox.js](https://github.com/mapbox/mapbox.js/blob/mb-pages/src/util.js#L32)