Provide a `Y.Model.Base` class which can be subclassed and provide all the features of `Y.Model` sans observable events. `Y.Model.Base` will extend `Y.BaseCore`.

`Y.Model.Observable` will also be provide as a class extension to mix in observable events to a model subclass.

`Y.Model` will have the features of the current version, it will be `Y.Model.Base` pre-mixed with `Y.Model.Observable`.

`Y.BaseCore` instance objects are ~70% smaller (in memory usage) than `Y.Base` instance objects. When using models on the server it is rare that events would be used since the lifecycle of the model
instances is so short (just for the request). It is also rarely the case that a model would listen for its own events. By splitting out a `Y.Model.Base` class, the slimmer version of the model class
could be used on the server, and when loaded on the client, the `Y.Model.Observable` extension could be automatically mixed in.