Supporting multiple models per page

It would be great if one could eventually author web pages that have multiple NetLogo Tortoise models. I tried to combine two on a page by creating and running startLoop on two different "sessions" (and using two different "model-container" ids). The second model was broken. Probably relates to #16 and #36.

This is definitely on the roadmap, but, for the time being, it can be worked around with iframes.

See lengthy discussion on #16