Make Model loading/saving/destroying consistent.

- Add isLoading(), isSaving(), and isDestroying() methods.
- Add isDestroyed() method.
- Add 'loadbegin', 'load', 'savebegin', 'save', and 'destroybegin' events (Model already had 'destroy' event).
- Change name of `reload` method to `load`.
Add build files in dist/, which require() all classes so that they are all initialized and can be used by their `type` names (such as for the attributes).