Added parseModels and parseModel to fix duplicate instances being created after create an update.

This fixes this bug while still maintaining backwards compatibility. This introduces `parseModel` and `parseModel` methods that are called by `update` and `create`, which keeps #301.  If they are provided, they will be used over a provided model and models method.

This is documented with newly added `@deprecated` tags in DocumentJS.  @tomgreever should make sure the styles look right before we launch.

This is for 2.1 because it deprecates some of `model` and `models` functionality.