Cleanup and add some docs.
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 branch 'master' of github.com:ahume/serviceworker-demo