Added gui support to the dockerfile.  Added a little explanation in the ...

...readme.
Love this one, missing things:
 * Make executable
 * Add python's shebang

Also for the previous helper script, what I would do is to add use documentation like:

```console
$ docker pull andresriancho/w3af:unstable
$ wget https://raw.githubusercontent.com/andresriancho/w3af/master/extras/helper-scripts/w3af_gui
$ chmod +x w3af_gui
$ ./w3af_gui
```

Please note that I renamed the script to w3af_gui to match the one we have in the "non-docker" installations, so users "feel like home".

Also, the link allows users to always download the latest from `master` (that's in the URL).