Destroy to empty

Regions have two very important functions that are inverses of one another, currently known as `show` and `destroy`. These methods work just fine, but the name of `destroy` can be a source of confusion, I think.

In every other class, `destroy` prepares the class for garbage collection. How surprised would you be to hear that a region's destroy doesn't do that, but instead prepares its view for gc by calling `destroy` on the view? This leads to strange language in the documentation like ["This will destroy the regions without removing them from the RegionManager instance."](https://github.com/marionettejs/backbone.marionette/blob/v2.0/docs/marionette.regionmanager.md#regionmanagerdestroyregions) Yikes. Talk about confusing.

Regions have no destroy method right now. They can only be cleaned up when they're being used in conjunction with a RegionManager. While we may want to change this in the future, for now we need to change the current `destroy` method to make more sense. I opted for `empty` in this PR, which makes reference to us, you know, emptying the view.