Refactor Region to sub-class Object

@samccone noticed two hours ago that `Region` was a simple plain vanilla js function. 
https://github.com/marionettejs/backbone.marionette/pull/1953/files#r18319779

After this PR, `Region` will become a true Marionette Object. 

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Is this PR backward compatable you ask... You betcha!
mmm...I'm not sure how I feel about the 2nd argument of extend. I'd rather it remain `_.extend(Region, {});`