Implementation of separate horizontal/vertical axes padding to the axis_grid toolkit

This feature is useful in cases such as showing the same section of an image with different colorbar ranges.
Since colorbars have to be labelled, it is not possible to get things right when using the same horizontal and vertical spacing.
I attach an image (you can obtain it by running the demo script or by generating the documentation, the new functionality is the image to the right).
![padding](https://f.cloud.github.com/assets/2823847/1222793/9b7eeb14-2702-11e3-82e5-c0c6588be055.jpg)

I think that I haven't broken anything since the documentation generation did not complain. However, I don't know how to write tests for this feature.