Add simple build script

@kentquirk @jh-bate would you like to take a look and tell me if this is ok for you?

Changes:

- The bundle and other temporary files for development now all go to `build/`, so they don't "pollute" the top-level directory
- You can now use `npm start` (which runs webpack in development and watch mode)
- Running `npm run build` (which delegates to a simple Shell script at the moment), will put everything needed for publishing into `dist/` and zip that into `dist.zip` ready for upload to the Chrome Web Store
- These instructions are in the README