Make it easy for users to snap axes to images

[astrofrog]

As mentioned by @cdeil in #115, currently calls to set_xlim and set_ylim are needed when we overplot contours for example. It would be nice to provide a convenience method that will snap the axes to a specific image size if needed rather than having to look up the dimensions of the image.

[astrofrog]

The normal Matplotlib way to do this is to simply call ax.set_autoscale_on(False) after plotting the image. Maybe this just needs to be better documented.

cc @Cadair