Rename Region#open to Region#setEl

When people see `Region#open` they often think that `open` is the main way to show a view. I think a better method name, would be one that suggests that its used internally to update the region's dom, thus `updateEl`. 

##### scope
I'm thinking this method should still be public because, others might want to override it for animations or more nuanced region work like maybe a different update strategy like `append` or `prepend`.

##### v2.0
I'd like to see both `Region#updateEl` and `Region#empty` go into a `v.2.0-pre2` release as they're api changes that fit into the Region#destroy and Region#show work. It's like an api bug, right?!?!