lazy load layers in an AMD and Lint-friendly way.

 All these layer types are supported:
- csv: CSVLayer // untested
- dynamic: ArcGISDynamicMapServiceLayer
- feature: FeatureLayer
- georss: GeoRSSLayer
- image: ArcGISImageServiceLayer
- kml: KMLLayer
- label: LabelLayer //untested
- mapimage: MapImageLayer //untested
- osm: OpenStreetMapLayer
- tiled: ArcGISTiledMapServiceLayer
- wms: WMSLayer
- wmts: WMTSLayer //untested