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.
I don't think so. We'll get rid of issues of errors when trying to install the same app on our different Marketplaces and be able to install apps that specify ```installs_allowed_from``` though.