Menu screen: Moving ground that starts moving when go is pressed, and also stops when stop is then pressed.
Also added custom button as imageview with button properties
Added FlappyBird! Flapping!
Added a gravity demonstration to main menu view controller
Instructions to see demonstration:
1. Click ‘Go!’ in order to get animations going
2. Click ‘Gravity’ to get the gravity going
3. Click anywhere other than the start button to make flappy bird fly!
Merge remote-tracking branch 'flappyBird2/master'