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 allows to update hue, saturation, brightness, contrast values.
ol.Map now have a LAYERGROUP property

The LAYERS property is gone.
Create a custom event type for vector layers

Vector layers will now dispatch ol.layer.VectorLayerEventType.ADD and
ol.layer.VectorLayerEventType.REMOVE event types instead of the generic
goog.events.EventType.CHANGE event type.
This will fix a maximum call stack size exceeded javascript error.