Push states in the browser history when changing day or track

[AurelioDeRosa]

When changing the day or the track visualized, the history of the browser should be updated. For example, if the user is browsing the track "JavaScript" of the 6th April 2015, the URL should be something like "/conference/04-06-2015/JavaScript". The URL allows the application to save its state that has two main benefits:

• Let the user access the tracks of a given day or a specific track of a given day by specifying a URL
• When going back in the history after reading the details of a speaker or a talk, the application will show the last day and track combination accessed

[AurelioDeRosa]

Fixed by commit a28a8a7