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.
Why don't you use function declarations

```
function iframe_install() {
}
```

instead of anonymous function expressions?

```
var iframe_install = function() {
};
```

By omitting the function name, you're actually creating a new variable called `iframe_install` that references an anonymous function.

If you're going to use function expressions, at least give the function a name:

```
function iframe_install() {
};
```

But at this point, you're typing more characters than had you just used a function declaration.

Also, look at this [interesting article](http://johntantalo.com/blog/dangers-of-anonymous-function-closures/) about the dangers of having anonymously named function expressions where you have closures later.

JSBins to illustrate:

* [anonymous function expression](http://jsbin.com/biver/1/)
* [named function expression](http://jsbin.com/gawav/1/)
* [function declaration](http://jsbin.com/cisuf/1/)