Added problem 15 description (Background)
Implemented Backdrop and FixedBackdrop classes
Moved screen resolution variables to Game class

Now screen width and height can be taken from the Game class by
Game::kScreenWidth and Game::kScreenHeight respectively.

Backdrop class uses this fields to fill in background.
Actually draw background using Backdrop

Map's method drawBackground() now calls backdrop draw() method. Game
class draws background, then Quote and then map itself.
Make black color of sprite transparent

Graphics class method loadImage now can be called with second optional
parameter black_is_transparent. When it is set to 'true', loaded image
would have transparent pixels instead of black pixels.