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

These layer types are supported and have been tested:
- csv: CSVLayer
- dynamic: ArcGISDynamicMapServiceLayer
- feature: FeatureLayer
- georss: GeoRSSLayer
- image: ArcGISImageServiceLayer
- kml: KMLLayer
- osm: OpenStreetMapLayer
- tiled: ArcGISTiledMapServiceLayer
- wms: WMSLayer

These layer types are supported but have not been tested and thus may require some modification:
- label: LabelLayer
- mapimage: MapImageLayer
- wmts: WMTSLayer