First attempt to have a proper sandbox cleanup on widgets stop
Improve mediator extension

- Keep track of events attached to sandboxes to be able to clean them.
- Add tests.
Add start and stop method in widget
Fix sandboxes tree
oops...
Adding spec for widgets nesting
use _.defaults for mediator default config + minor stuff
minor stuff
Merge pull request #206 from sbellity/feature/stop

Adding support for widgets nesting + proper disposal of listeners when a widget is stopped