Implement importScripts function in worker global.

- [x] Add importScripts function which sync loads external scripts.
- [x] Parse worker.js and wrap calls to importScripts in eval().
- [x] Figure out how to get importScripts to fail if called outside of initial run context.
- [x] Figure out how to actually throw reasonable error when called outside initial run context.
- [x] Better error handling when requesting/reading importedFile.
- [ ] What should happen when URL (or one of multiple URLs) fails to load?
- [x] How can worker/importScripts() be aware of the current absolute URL of the worker?

Address #5