For issue #48

This is for issue #48. Currently this downloads the larger cube 'L1448_13CO.fits' because that's the file used in the image tests in the main repository. 
Also, since this is the first time that users will see the use of the datasets module, it might be worth mentioning below this example that the ``hdu = datasets.msx_hdu()`` is equivalent to:

```
from astropy.io import fits
hdu = fits.open('msx.fits')[0]
```

Just so they understand what it is doing.