Allow loading the "chrome" module

Per the discussion here (https://groups.google.com/forum/?pli=1#!topic/slimerjs/xGmEyvUIf90).

There are some test failures, but I think they are unrelated to this change. They are:
- system module should have a os object
- webpage with listeners should generate the expected trace for the error page
- webpage.sendEvent send keypress event with alt + keycode of a non-printable char
- WebPage.render() can capture and save it as PDF into a file
- WebPage.onError is called when an error occured during js inclusion