Added layer logic to use current viewspace in queries

Details documented in https://github.com/jackiekazil/geoq/issues/26
>Added support for ClusterLayers and used that with some small extentions to ESRI Leaflet to support images from ArcGIS service. Have tested with both CAP images and Disaster Reporter. Some additional work may need to be done on how/what is displayed in the marker popup along with further customization of the marker.