Add support for getEmptyView [#678]

I implemented the feature proposed in https://github.com/marionettejs/backbone.marionette/issues/678

I think it's useful because:
* allows for customization of emptyView
* helps with some dependency loading issues
* we have getItemView already, makes sense to have getEmptyView

for your consideration.

PS. Thanks for all the great work on marionette! I <3 it ;)