call modal.destroy() in modalmanager destroyModal()

I upgraded my copy of your library to the latest master version. I then found that when I opened and closed my modal without reloading the page, I wasn't receiving the .on('show') events that I had attached to the modal instance. After looking at the diff of changes, I noticed that modal.destroy() wasn't being called anymore. Adding that back fixed the issue for me.

See the screenshot of the diff that I'm talking about: http://take.ms/wzZmh