Release 3.2.0

* Remove Xhtml.
* Remove plus elements.
  For example, table doesn't enforce non-emptyness anymore.
* Add various types that were not exported (img, dl, figure, rp, rt and ruby types).
* Expose `string_of_number`, the better stringifier introduced in 3.1.0.

* Functorial interface breaking changes:
  * Modify the functorized interface to export typed events.
    You can now specify different handlers for keyboard and mouse events.
    There are two new types and functions (keyboard and mous events) in the Xml signature.
  * Add a wrapping type for lists of nodes. See reactiveData and new eliom wrapping.