Create web app target for axiom-shell.

To create and run axiom-shell as a web-app, build the "run_web_app" target, then point Chrome to http://127.0.0.1:8282/main_window.html.
This chrome/background module was specifically meant for use as a chrome app background page.  I'd suggest either a new 'axiom_shell/web/main' module, or just put the important bits of background.js directly into web_app_startup.js.