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.
this might get confusing considering how we have [`user.has_installed`](https://github.com/mozilla/fireplace/blob/master/hearth/media/js/commonplace/user.js#L76-L78)