axes_grid1: ImageGrid respect the aspect ratio of axes. 

I thought I committed this change a while ago. There is a reference to my commit 

https://github.com/matplotlib/matplotlib/commit/f6b344e

but somehow this is not shown in the commit history of the master branch (nor the release branch).

This PR is to resolve #955, which I closed sometime ago since I thought I made this commit.
I am making this PR against the v1.3.x branch as this is more like a bug fix, but if others think this is not appropriate, I will rebase and make another PR.