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.
you're not going check the origin? some rogue site could `<iframe>` us and `postMessage` to install some package that the user didn't ask for.