Blanced game and modified start/gameover screens.

Changed a few constants in constants.h as per Paul's recommendations. Now
the game is more balanced/fun to play. I also modified the preGame and
postGame screens -- I added simple instructions on how to play at the
beginning, instructions on how to restart after the player has lost, and a
new gameover screen where it tells the player if he/she got a high score and
displays the overall best score at all times.
Merge branch 'master' of https://github.com/DevelopersGuild/flappybird
Updated Features.txt