Update collision detection code

Update PipePair.collides_with() to use Rects
Change collision code in main() to use any()
Move Game Over message
Add and use new Bird class, move some constants

All bird positioning and processing code has been moved into a new Bird
class.
Some constants have been moved into the Bird and PipePair classes, and
their usage has been updated.
For simplicity's sake, PipePair.collides_with() now accepts the bird's
rect instead of its position.