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.
well the Marketplace deals with only apps. so are we going to change `ratings.js` to `apps_ratings.js`, `previews.js` to `apps_previews.js`, etc.?