Patching in AMD compatibility

I needed to use ical.js as an AMD module, so I patched in the support for my copy. If this is something you're interested in adding to the official release, please feel free!  This adds the ability to use ical.js in the following ways:

- globally (fixes the "exports is undefined" issue when included in HTML)
- as an AMD module
- as a commonJS module (as in a nodejs environment)