Added partial support for image services

Includes:
- Added _serializeImage() and use that when declared class is esri.layers.ArcGISImageServiceLayer
- Added support for image service rendering rules, still need to add bandIds, mosaic rule, etc
- Moved common code for dynamic and image layers to _serializeDynamicCommon()
- Added 2 example pages: image service, and rendering rule
- Updated index.html w/ links to new pages and README w/ newly supported layer types

You can see updates live here: http://tomwayson.github.io/cereal/