default layers:'' param

I've added default layers parameter. Even though this is more of a implementation fix (ex. L.agsDynamicLayer(dynUrl2, map.getBounds(), {layers: 'show:1,2,3,4'}); ) it would be beneficial to have it as a default param which can be overridden in case you're using a map service with multiple layers. It's more of a mental note for devs. To bad there is no show:all parameters to show all layers present. 

Also, when adding a layer check for map projection and update default values for bboxSR and imageSR. This fixes bug where on map panning image starts moving in same direction