(#1014) - visualizeRevTree uses only public API

VisualizeRevTree now uses only public API so it should not get broken after some internal Pouchdb changes. I still like to as an educational tool which will hopefully help people understand rev tree quickly so it's nice to have it around but with #1012 in mind this change lets this plugin be totally core independent.

Do we want this utility to be somewhere near http://pouchdb.com/api.html or http://pouchdb.com/external.html in fact? Any ideas of how to do it more useable/better/whatever?