Bug fix/update app bg

Fixing bug, where Marathon would not update running tasks in app list in background when changing number of instances in app modal:

[![Marathon showing more running tasks than actually running](http://cl.ly/image/1x1D0h0x180s/Image%202014-08-25%20at%2014.03.19.png)](http://cl.ly/image/1x1D0h0x180s/Image%202014-08-25%20at%2014.03.19.png)
This is getting hairy and a reason I understand the motivation behind something like Flux: who owns this model? The AppListComponent has a reference to the list, and this modal is handed a reference that it is now modifying. I'd rather see the polling moved higher up the component hierarchy so the modal is not modifying any models.