`el: false` multiple top level (officially not supporting) in 0.8.*

Display warnings if using el: false with multiple top level elements, but allow suppression if the end user knows what's up.

This will close https://github.com/tbranyen/backbone.layoutmanager/issues/314 & https://github.com/tbranyen/backbone.layoutmanager/issues/319

This pull request still requires testing and documentation.