fix : reduce piskel model size

- added backward compatible implementation for v1 models
fix : reduce piskel model size

+ piskel deserialization is now clearly asynchronous
+ added utils.Deserializer (not a singleton though, more a builder/loader)
+ utils.Deserializer constructor expects a callback
+ when all layers are loaded and piskel is ready, the callback provided by
  the client is called with piskel as the first argument
- Deserializer doesn't fit in the utils package, which should be reserved
  to singletons : can move it to service as a PiskelLoaderService, and
  Deserializer could remain with only the purely static methods
- ImportController is realying on the Deserializer to build a Piskel but
  it shouldn't. Find a way to mutualize the code necessary to create a
  Piskel from an array of pskl.model.Frame
- still cleanup to do in app.js
- comments to add as well