Atrament v4.1.0

This version should fix #98 - it changes how the dirty flag behaves, as it's now toggled in the draw() method. This means it will also be true if drawing programmatically, not just when using a pointer (mouse/touch/stylus).

Another change is that the clear() method now clears the canvas regardless of whether Atrament has drawn on it. I believe this will lead to more predictable behaviour. The method has also been refactored to take into account the canvas transform.