Fixed pane assignment to work with 0.8 pane management features.

The docs stated that you could now set the Pane for the labels, but this was not the case.  Currently, the labels go on the marker pane if the target object is a marker, otherwise, the popup pane.

This pull request fixes this.  You can now specify `pane: "myCustomPane"` in the label options and the label will be added to that pane, otherwise if no pane is specified, it follows the current rules (if the target object is a Marker, the label goes on the marker pane, otherwise it goes on the popup pane).  This is made to work with the current 0.8-dev pane management features of Leaflet.

I also went ahead and included the built sources as well.