use apps iframe for installs/launch (bug 1029687)

Use the iframe to:

- install apps
- query for installed apps
- launch apps

In market_button.html, check if the app is installed to render the right text rather than asynchronously updating the button after the app is rendered.
it confused me though, so maybe `helpers_fireplace` is a better name or something. or just `helpers.js` and have `commonplace/helpers.js` be the commonplace-specific one.