Permit layer stamping to be overridden

Extracts the detail of how layers are stamped to a single method which can be overridden. mapbox.js would use this in [markerLayer](https://github.com/mapbox/mapbox.js/blob/v1/src/marker_layer.js) to implement a [key](http://mapbox.com/mapbox.js/api/v0.6.7/#markers.key) function.

I suspect it will also be useful towards #1416 -- in general, it's useful to be able to set up a GeoJSON layer group that associates features and layers based on the feature's `id` property rather than an arbitrary monotonic stamp.