surface definitions fixed ( surface -> self.surface )

Fixed the following errors, and hence the crashing of the game:
Traceback (most recent call last):
  File "flappybird.py", line 266, in <module>
    main()
  File "flappybird.py", line 209, in main
    pp = random_pipe_pair(images['pipe-end'], images['pipe-body'])
NameError: global name 'random_pipe_pair' is not defined

-----------------------------------------------------------------

Traceback (most recent call last):
  File "flappybird.py", line 266, in <module>
    main()
  File "flappybird.py", line 209, in main
    pp = PipePair(images['pipe-end'], images['pipe-body'])
  File "flappybird.py", line 73, in __init__
    surface.blit(pipe_end_img, (0, bottom_pipe_end_y - PIPE_PIECE_HEIGHT))
NameError: global name 'surface' is not defined