support for firefox open web app

With this pull request the web interface check if the browser is Firefox based and support the Open Web App standard.
In this case a popup was showed that ask to the user if he want install the app as an hosted webapp.
This system is supported on Firefox Desktop, Firefox for Android and Firefox OS.
In this way the web interface is saved like a native app in Firefox Desktop and Firefox for Android while in Firefox OS is an app.

[Homescreen](http://i.imgur.com/mbvXefB.png)
[App](http://i.imgur.com/sEdNxEF)

I don't know the web interface, but i think that a system for check (with localstorage or other solution) for not ask at every refresh of the page if the user want install the app is needed.