Refactor dynamic script loading. Add util.js:load_scripts()

Related to issue/pulls:
https://github.com/kanaka/noVNC/issues/194
https://github.com/kanaka/noVNC/pull/201
https://github.com/kanaka/noVNC/pull/202

In IE9, the window.onload event can fire before dynamically loaded
scripts have finished loading. This can result in either WebSocket (in
the case of vnc_auto.html) or RFB (in the case of vnc.html) not being
defined at the point when window.onload is called.

- Move the load_scripts routine from vnc.js to util.js (so that
  websockify can use it too). Also, refactor to work when load_scripts
  is called by a script that itself uses load_scripts. When the whole
  chain of dynamically loaded scripts is finished then call
  window.onscriptsload. Use this mechanism in all the places that
  depend on dynamic loading of scripts: vnc.html, vnc_auto.html,
  websock.js, tests/vnc_playback.html, and tests/vnc_perf.html.

- Use the new window.onscriptsload handler instead of window.onload.

- Remove include/start.js and do the script loading and startup event
  handling in include/ui.js instead.