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. 
@anizami - could you not do

```
from astropy import datasets
cube_hdu = datasets.l1448_co_hdu()
image = cube_hdu.image
w = WCS(cube_hdu.header)
```

? I think it would be nicer to always return the HDU for consistency.