[FIX] compile of rst module descriptions to HTML

lxml (used by the html sanitizer) really does not like documents as unicode
strings with an encoding declaration (which doesn't make sense indeed), yet
docutils with output_encoding=unicode generates one (a stupid one too, says
"encoding=unicode").

Disable the xml declaration altogether when compiling rST module declarations
to HTML.
Merge pull request #998 from xmo-odoo/8.0-module-descriptions-xmo

Don't include xml declaration when compiling rST module descriptions to HTML