Marionette 2.* breaking change: 'close' is now 'destroy'

Marionette 2.* will now "destroy" a view rather than "close" it, and so triggers a "destroy" event rather than "close".

I've added a `destroyWith` helper and `destroy_with` option that parallel `closeWith` and `close_with`.  Rather than trying to do some fancy versioning thing (e.g. could check for existence of Marionette.VERSION in order to decide which to define) I've taken the simple approach of providing both and leaving it up to users to choose the appropriate one.