Add support for WMS layers, non-base tile layers, deactivated layers and layer opacity

- Add L.TileLayer.WMS options to BaseLayer so that WMS layers can
  be used as base layers.
- Implement L.TileLayer and L.TileLayer.WMS as components so that
  they can be added to map, layer groups, etc. and not only as
  base layer.
- Add layer opacity option.
- Add "active" option (default true) to AbstractLeafletComponentState
  as a means to create layers without adding them to the map (e.g.
  add them to the layers control only, thus initially being hidden
  with the user being able to activate them on-demand). Correct
  solution would be to re-think the layers control in v-leaflet so
  it wraps all the addOverlay, add... functionality.
- Add 'LayersTest' page with examples for all then changes.
Merge pull request #7 from jtreml/add-wms-support

Add support for WMS layers, non-base tile layers, deactivated layers and...