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
Don't allow importScripts to be called once worker code has been executed.
Let importScripts support multiple URLs as args.
Throw error so it logs to console correctly.
Merge pull request #31 from ahume/master

Implement importScripts function in worker global.