524 fix aufs links related warnings

This PR fixes #524.

The first commit modifies mount.go so that we try to run ```auplink``` before unmounting the aufs layers. If docker fails to run auplink, it will print a warning and move on.
The warning looks something along these lines:
```[warning]: couldn't run auplink before unmount: couldn't find auplink in $PATH```

The second commit adds aufs-tools to the list of dependencies for the installation of lxc-docker on Ubuntu and Debian.

```auplink``` makes sure that hard links across layers are handled properly upon unmount.

@creack Please take a look at this PR.