Change Draft-mark handling

 1. Replace textual mark by whole-page bitmapped image.
    This is necessary to avoid accidental selection of the mark
    when copy&paste-ing from the document.
 2. Add a draft-indicator at bottom and top of the page.

Eventually, we can:
 1. Remove the watermark altogether
 2. Remove the draft-info from the normal page footer.
Merge pull request #36 from krono/krono/draft-enhanchement

Change Draft-mark handling