FlxAngle: Added angleFromFacing. FlxVelocity: DRY

Accelerate functions now take a single maxSpeed (instead of a maxSpeed separately in x/y)
You would set the X and Y speeds to .707 if you wanted a scalar speed of 1.  Out of convenience, though, I'd rather tell it a scalar speed and have it sort out the vector X and Y components. When would you ever restrict the speeds differently?