Changed model.activate and model.deactivate to private methods by
denoting them as `._activate` and `._deactivate`.
This is to discourage use of these methods directly. They are private
methods intended to only be called by stateful-collection. I also
realized that it would be big trouble in little china if users deviated
from our `model.active()` and `collection.deactivateAll()` apis and started
throwing in their own `model.set('active', true)` or `model.set({active:
false})` type calls. We will need to discourage that very plainly in the
documentation.