Fix offline Q&A when running as firefox app

Fixes #173.

assets/javascripts/app/app.coffee

@@ -87,6 +87,7 @@
     @hideLoading()
     @welcomeBack() unless @doc
     @removeEvent 'ready bootError'
+    navigator.mozApps?.getSelf().onsuccess = -> app.mozApp = true
     return
 
   initDoc: (doc) ->

assets/javascripts/templates/pages/offline_tmpl.coffee

@@ -43,6 +43,8 @@ canICloseTheTab = ->
   if app.AppCache.isEnabled()
     """ Yes! Even offline, you can open a new tab, go to <a href="http://devdocs.io">devdocs.io</a>, and everything will work as if you were online (provided you installed all the documentations you want to use beforehand).<br>
         Note that loading any page other than <a href="http://devdocs.io">devdocs.io</a> directly won't work (due to limitations in AppCache). """
+  else if app.mozApp
+    """ Yes! Even offline, you can open the app and everything will work as if you were online (provided you installed all the documentations you want to use beforehand). """
   else
     """ No. AppCache isn't available in your browser (or is disabled) so loading <a href="http://devdocs.io">devdocs.io</a> offline won't work.<br>
         The current tab will continue to work, though (provided you installed all the documentations you want to use beforehand). """