Allow user to specify an axis instead of a figure

I found myself wanting to use matplotlib/pylab's built-in `subplot` routine and then have aplpy plot on one of those axes, so I tried implementing that. Unfortunately, aplpy requires a `ParasiteAxes`/`HostAxes` instance.  I tried creating one (see commented out code), but right now this just raises an exception if the wrong type of axis is specified.

This may need to be rebased against the latest astropy refactor...