show_contour() plots the FITSFigure image

I ran into a situation where memory issues dictated being able to have show_contour() use the image from FITSFigure, as described in Issue #123. That is,
```

f = FITSFigure('image.fits')
f.show_contour()
```
produces a contour plot in the way that currently would be achieved with

```

f = FITSFigure('image.fits')
f.show_contour('image.fits')
```
The following example
```

f = FITSFigure('image1.fits')
f.show_contour('image2.fits')
```
works in the same way that show_contour() currently works. I've tested it out in a handful of different scenarios where I've used show_contour() in the past, and I have not encountered any problems.