show() functionality

This wraps a couple more functions: `fits_file_mode()` and `fits_get_hdu_num()`.
It also overrides `show()` so you now get:

```julia
julia> f = fits_open_file("alpha_lyr_stis_003.fits")
file: alpha_lyr_stis_003.fits
mode: READONLY
extnum hdutype         hduname
1      image_hdu        
2      binary_table    'SCI     '
```
(I'm not sure you want this behavior in ``show``; could move it to a different function)

I also fixed what I think was a bug in `fits_file_name()`. 

I just started with Julia (and I'm no cfitsio expert either) so I'd appreciate any feedback!