Provide a method to dispose an Aurelia instance

The framework should provide a method to shut down / dispose an Aurelia application instance.

Calling `aurelia.dispose()` would dispose all objects / event listeners created by Aurelia including:
- unbind/detach views
- release instances held by containers
- dispose all EventAggregators and bindings to release subscribers
- dispose / deactivate the router(s)
- null out the aurelia.container property and the aurelia.loader properties (as recommended by @EisenbergEffect in a private Gitter chat about this)