Fire zoomlevelschange event when zoomlevels are updated.
This is triggered when you remove a layer from a map with greater
zoom level coverage than the remainding layers or when you add a tilelayer
with greater zoomlevel coverage than the previous set of layers had.
Add tests for the zoomlevelschanged-event.
Don't expose getZoomLevels.
Merge pull request #1376 from moonlite/zoomlevelschange-event

Zoomlevelschange event