Fifth Commit - The Hungry Fox.
- Extended the xml file (history.xml). Now it contains a background tag
with a string to a background file to load for each part.
- Update Assets, MainScreen to use this update.
- Made a new class: TalePart, which contains the feature for each part 
of the tale. Now it only stores the several backgrounds and draw them by 
the index part. So, in MainScreen class I call this method each times 
with the different index incremented as soon as I change part!
- Added a FPSCounter class to evaluate performances (still 60 fps).
- Started the same thing of backgrounds with animations.

With this update to add background at the tales users can only change 
the xml file!