Add activate & deactivate methods

Note that this PR is meant for discussion purposes and is not ready to be merged yet. Summary:

- removed `freeze`
- added `activate`
- added `deactivate`
- added `_removeChild`
- override `remove` to call `deactivate`

Questions: 

- `undelegateEvents` is no longer called, should it be re-added to destroy? Does not seem appropriate for `deactivate` as an unattached `el` should not receive any DOM events and allows for an easier re-`activate`
- what problems will `activate` and `deactivate` propagating to children cause?
- `deactivate` currently calls `stopListening`, but there is no simple way to get `activate` to re-add all of the events `listenTo` previously registered. Ideally calling `deactivate` and `activate` in success would keep the view in an identical state prior to those methods being called.