Make the SVG sigs wrappable in a monad.
Make the html wrappable in a monad.
Tying up the knot in order to makes everything work with the wrapping monad.

The wraping monad is now in a separate module. The Xml module contains
only the type 'a wrap. Functors in Html_f and Svg_f are specialized
accordingly. Each functor have a partialy applied version against the
empty monad.
Add documentation for the functorial interface of tyxml.
Add a short documentation for MakeWrapped functors in the .mli.
Whitespace changes, to fix the damages done by regexs.