make toggle button work on edX


changelog

  2014-05-12 (MrJ):
    renamed CSS class from 'minimized' to 'enabled' (opposite semantics)
    removed faux clicks and non-functional pre-ready-state DOM checking
    moved premature iframe loading into DOM-ready callback (this was the main bug)
    changed the default state back to disabled (to address legitimate privacy concerns)
    chat is truly disabled by default and is not simply hidden as previous versions
    added "opt-in" button to trigger the actual chat frame loading and login
    changed README.md to instruct loading './app/chatframe.js' to avoid updating xBlock
    much refactoring , reformatting , and a few minor CSS tweaks