Change how includes/excludes are managed with duplicity.

The previous version didn't allow this kind of usage:
backup /var/foo
exclude /var/foo/bar

This is due to duplicity taking includes and excludes in a not-so-smart
way.

What changes now:
- "includes" list is replaced by "directories", which is also a list.
  You *must* prepend entries with either a "+" or a "-" in order to tell
  duplicity what to do
- "excludes" list is removed

If we take the previous example, the entry will look like:
duplicity::backup {'foo':
  …
  directories => ['- /var/foo/bar', '+ /var/foo'],
}

This will be the same as:
duplicity … --exclude='/var/foo/bar' --include='/var/foo'

Please update your duplicity::backup calls before breaking all your
backup system ;).