ADD clickable option to FeatureGroup/GeoJSON

This is implement the second of the three optimization that I referred to the first comment on [this blog post](http://leafletjs.com/2012/10/25/leaflet-0-4-5-bugfix-release-and-plans-for-0.5.html). These aim at optimizing the handling of geoJSON layers with thousands of sublayers.

Namely it adds a clickable option to LayerGroup & GeoJSON, that disables attaching DOM event handlers for layers that we do not intent to handle mouse events.

PR with other mods and example will follow