Clean README further.
Fix installed/installing typo.
Add note about the example site.
Move postMessage into WorkerRegistation.

This also removes worker > page postMessage as it's not defined
very well. Close #30.
Compare worker files before executing the new one.
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.