default imshow extent

Some discussion (and/or a ruling by John) is needed. My

    > preference is that if a change is made, it be a change in
    > basic default behavior, not additional options, because any
    > non-default behavior can be achieved easily using the extent
    > kwarg in its present form, and at some point adding options
    > and more complicated handling of kwargs causes more trouble
    > (complex code and documentation) than it is worth. I don't
    > see any advantage of the present behavior over the
    > matlab-equivalent behavior--but that may be because I am
    > ignorant of the original rationale.

I'm weakly inclined to leave things as they are, since it is so easy
to get the desired behavior by setting the extent kwarg. If we just
put a help string in the imshow docstring telling people how to get
what they want, that may suffice. That said, I am a light user of
images and do not feel strongly at all. If people who are heavy image
users feel strongly, I'm happy to change. Perry?

    > If imshow behavior is changed, a corresponding change will
    > be needed in the image-matching capability of contour.
    > Maybe matshow would also change; I haven't checked.

matshow doesn't really have much of a purpose any more. It's primary
reason for being was to handle aspect ratio when aspect ratio was
broken. Fernando -- do you see any reason to keep matshow?