Fixed pane assignment to work with 0.8 pane management features.

The docs state that you can use the `pane` option to select a custom pane for your label. This is incorrect with the current source. Currently, the labels go on the marker pane if the target object is a marker, otherwise, the popup pane.

This pull request fixes this to work with the new pane management features of the 0.8-dev branch of Leaflet..  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, if the object is a marker, the label goes on the same pane as the marker.  If it's not a marker, then the label goes on the `popupPane`.

I also went ahead and included the built sources as well.