Hooks can now be dynamically and multiply defined. Using symbols for definitions.
Work around apparent RubyMotion issue with redefine classes in spec.

If this class is named Task, the spec fails as the hook methods are not being called. Unclear as to why, but it is doubtful this would be an issue in a real app.
Fix Column#classify for :has_one
Add Model#_destroy accessor, to mark instances for later destruction