Jango service integration

First functional version of Jango service integration for Nuvola Player 3

Features of previous integration for Nuvola 2 are working: play / pause / thumb up / thumb down / song info and art.
I cannot find how to work on this. I never create a `track` object with undefined values, as the variables `song`, `artist`, `album` and `album_art` are inizialized with `null`.