Use a mechanism described here to serialize the execution of scripts
under IE: http://wiki.whatwg.org/wiki/Dynamic_Script_Execution_Order
Basically, we set the src attribute but do not append the script to
the body/head until after readyState is "loaded". Unfortunately, Opera
uses readyState but in a different way (always "loaded" for both load
and execution).

This is related to (and hopefully fixes) these issues:
https://github.com/kanaka/noVNC/issues/194,
https://github.com/kanaka/noVNC/issues/205,
https://github.com/kanaka/noVNC/issues/208.