Display ArcGIS REST layers

New ArcRestSource plugin (gxp_arcrestsource) for displaying layers from ArcGIS REST servers.  

Requires OpenLayers.Layer.ArcGIS93Rest.js

Modified plugins/AddLayers.js and widgets/NewSourceDialog.js: Added a dropdown for selecting either WMS or ArcGIS REST as the source type (default is WMS).

For PR https://github.com/GeoNode/geonode/issues/268