Generalize survey question creation

This abstracts the process of creating survey questions. Instead of directly writing HTML, you make a Question (either a Fixed, Scale, or Essay Question) and pass in a bunch of parameters. The survey-gen.js classes then do all of the work of setting up DOM nodes.

This implements a basic Fixed question. Scale and Essay questions are still TODO, but the groundwork is here.
Are you planning to localize the extension? Even if not, it might be better to put this string and others to  _locales/en/messages.json and use chrome.i18n API to read it.