Change to aspect ratio handling

Hello Eric,

First of all: great job done with imshow()! works great!
When playing around with it we noticed a little problem propably due to a
rounding error. It is quite good to see when using a backend like GTK (axes
aren't antialiased there).
When running the code below you can see a white line at the bottom
which won't disappear even when moving the plot.

import matplotlib
from pylab import show, imshow, rand

z = rand(20,20)

So again: great job with the aspectratio but I just wanted to state this one.

Nikolai and Martin