Implement importScripts function in worker global.

* Add importScripts function which sync loads external scripts.
* Parse worker.js and wrap calls to importScripts in eval().

Address #5

TODO:
* Figure out how to get importScripts to throw if called outside of initial run context.
* Better error handling when requesting/reading importedFile.
* How can worker/importScripts() be aware of the current absolute URL of the worker?