Add autoscale

Display is now able to autoscale. By default there are a few scale values defined (0.2, 0.5, 1, 1.5 and 2 A) but you can adjust these by just changing the array.

The drawing routine is now less efficient time-wise, since I now store the full scale previous readings in the buffer, and only scale them to 24-0 at draw time. The reason for this, is that when scaling down, this preserves the resolution of previous readings which would be lost otherwise. And the sketch is fast enough for supporting this.

Also, now displays current reading with justification.

Please test!