adds map bounds aware `LayerGroup` rendering.

`LayerGroup` manages child vectors and markers to only add to the map those within map bounds at map `moveend`, others outside map bounds are removed from map.

For performance improvement. Saves browser/DOM from having to render elements not within current map view. Use case would be a data set with thousands of features (maybe single GeoJSON file) added to a LayerGroup. Zoomed to a reasonable level, only 10, 20, 100, whatever are ever within the map bounds at one time. Preferably only those 10, 20, 100, whatever should be added to the map (to the DOM).

Could also have option to disable. I don't see a case where disabling would be advantageous, but could be an option in case.

[Resulting interaction with LayerGroup with Markers](https://f.cloud.github.com/assets/1182714/1213331/78daf048-2627-11e3-84a9-9f9bb9599279.gif)