Merge pull request #797 from bbinet/layergroup

Layer group support
Make layer renderers more stupid

Layer renderers should not be responsible for listening to layer
properties change and triggering a render.
Layer change events are now forwarded to the map which will trigger a render.
Update color matrices based on framestate values

In getColorMatrix method, we'll update the color matrices only if layerstate
color properties has changed in the framestate.
Trigger CHANGE event only when required

For example only when layer is both ready and visible.