Update Flappy_Bird.pde

Renamed some variables, made classes capital.
Removed "public" keywords, unnecessary in processing.
used color variable type instead of int[] col
implemented Bird.pipesPassed counter for score (kind of awkward at the moment)
Removed redundant function call to gameOver.
changed Stage.checkCollisions and Pipe.checkCollisions to return booleans.
Simplified Stage.checkCollisions to only check corners. (may need to rethink if we want it to be more bird-shaped).