More work on obstacles

Obstacles now form from the top and bottom
Obstacles are now continually generated off-screen at a set time period
Obstacles are deleted and removed once to go off-screen again
Methods to detect collisions with obstacles added
Score is added as the player passes through the obstacles
Score is set to 0 when the player collides with the obstacles, a proper
Game Over state is yet to be implemented
Rewrote the draw method, can now just layer characters on top of each
other, makes it simpler to add new objects to the stage