Fixed drifting bug + couple cleanups

-Added IsColliding function to Character class
-Restricted jumping so double jumping is impossible - a temporary method
for the moment
-Stopped player randomly drifting left across the floor
-Note: this sometimes happens when you press the RIGHT button - WTF?!
-Probably related: stop player from travelling faster left to right than
right to left
-These were problems because of bit shift negative bias.