Ability to toggle visibility of ArcGIS layers in ArcGisImageryProvider

Requested on the forum: https://groups.google.com/forum/#!topic/cesium-dev/Y6PBcx3eB1Y

> ArcGisMapServerImageryProvider constructor needs a object which contains a property 'layer'. If I assign the layer attribute to '[0,1,2,4]', then I will get the correspoding layers in this arcgis server map service. Now I want to control the '0,1,2,4' layer individualy rather than controlling the visibility of this whole map service.
After I create a ArcGisMapServerImageryProvider instance, I can only read layers property and cannot assign it to a new array to change the layer visibility(not the mapserver visibility).

Read the ArcGisMapServerImageryProvider constructor and the linked forum request first. Define how individual layer visibility should be changed after provider creation, then verify that selected layers can be toggled without changing whole map-server visibility.

Written by the indexing model from the issue text.