Support figure / image alignment

The default docutils rst2html renders image alignments using a css-class (and the css-definition in the document head).
The creole implementation strips away the css-class, so we have no alignment information left.

I've implemented a fix which puts the alignment information in the html `align` attribute of the `img` tag,
since I think using css is not the creole way.
It's not too beautiful but it works, better ideas are appreciated ;)

I didn't figure what to do with the cross conversion stuff, so i didn't do anything there.

On github, which seems to use creole's rest2html, image alignment currently does not work, as reported in github/markup#163 - it would be great to get this fixed :-)