Add layergroup support
Add layergroup exports
Add tests for ol.layer.LayerGroup
Add tests for layer.getLayerState()
Add a layer-group example
Get opacity from layerState in webglmaprenderer

instead of getting it from layer.getOpacity().
Fix the issue of changing opacity which didn't work with LayerGroups in webgl.
Bind more layer properties in layer-group example

This allow to update hue, saturation, brightness, contrast values.