arrow keys (L & R) to move left and right Z to jump. You can "flutter" by holding Z for a little while after jumping. (This is really just an animation bug, and is purely cosmetic.)
Version 0.4 Added: You don't fall like a rock if you jump while moving. In fact, you jump higher than when at a standstill. Took like an extra month to finish, just because of lack of motivation... safe to say this is the last version here, at least for a while. This is more or less an engine rather than a platformer, but it's a platformer engine so in my mind it counts. It's possible to fall beneath the floor using a wall, but you need both speed and luck, and due to the way the engine works, you'll be spat back out above where you clipped.