Rake/ghpages: create Dash docset from manual

When running 'rake ghpages' the messages from the XSLT stylsheet
are converted to a dash docset.

See http://kapeli.com/dash for more info.
xpath: min/max functions
Bugfix: box dimensions on images/clip=yes

When clip=yes with images, the resulting image box had very incorrect dimensions. This is fixed with simple (negative) glue added to the top and
left.

Always use higher precision when rounding!