Adding Timber::ActivityNotifier & Refactoring Timber#subscribe

This commit makes the following updates to Timber:

- **Addition of `Timber::ActivityNotifier`**:   This is a mixin that must be added to ApplicationControllers to activate activity logging. It mixes in an after_filter, called `trigger_activity_logging`, that has two responsibilities:
    - Collecting data that should be passed to the activity logger
    - Triggers activity for the specific action that has been hit,  after the action/view has been rendered.

- **Refactors to Timber specs**:  Updates to the Timber specs given the above changes.