Added title property to app module

I've added a Title property to the app module.
This property is used by Durandal as the default title for messageboxes.
Also it is used to set the document title so we have a title before the router sets it.

Furthermore one could use this property to bind it to an html element. For example to bind it to the navbar > brand > span as seen in the Durandal starterkit.

With this title property one can reduce the number of places where the actual app title is declared. I reduced it from 5 times to only 2 (setting app.title in main.js and having it in the _splash.html)