Remove references to el and $el when destroying a view

Fixes https://github.com/marionettejs/backbone.marionette/issues/1010

I have branched from @ccamarat branch to rename close to destroy so would be good to merge that first I think.

Also been thinking about whether we should remove references to other properties like ```model``` or even ```collection``` in collectionview? Keen to see what you guys think about that, is it a potential issue?