add hasView property to region

Fixes #1632

An alternative solution would be to declare `hasView` as a function and return a boolean based on whether `currentView` is set.

thoughts @marionettejs/marionette-core?
I like the method because it's explicit about its usage and beneficial to newcomers wrapping their minds around Marionette's constructs.  Also, I vote for a `!!this.currentView` implementation just to avoid unnecessary extra state.