I've added a ButtonPressed function to the joystick class

I've wished that this was already in WPILib more than once.
The ButtonPressed function just checks if, the last time it was called, the button was false, and returns true if the button is pressed the current iteration.
It's useful for if you want to run things just once when a button is pressed (at the instant the code realizes it is pressed), but dont want to create a Button object.