Added Input::isAnyKeyDown

It basically looks at all the key states and returns true on the first
pressed key it finds. It's naive because if no keys are held down it has
to go through SDL_NUM_SCANCODES elements, generally EVERY tick. This
could use some work; maybe tracking the number of keys held down,
updating based on events rather than searching stored key states. That
way we'd just have to check whether the # of keys held simultaneously is
0 or not.

Ideally, I would also like to use Input::isKeyDown(ANY_KEY) to figure
out whether any key was pressed but implementing it this way would have
taken more effort than it was worth.