Source offset

Upper limits for axes and imshow extent were also off by 1.  Also round cutout boundaries before creating the cutout coordinate converter.  

Checked the results of these adjustments by viewing the same cutout in the matplotlib viewer and ds9.  ds9 only goes to 0.5 pixel resolution so it is hard to compare exactly, but as far as I can tell they are now matching up on the test image I used.  Again, it would be good for someone else who has done lots of blinking to double check, and I'll let them close the relevant issue (#166) if they think it looks good.