tools(xml_tools): fixed `<div>` unwrapping + added `parse` instance: `<div>` unwrapping could fail when a text node was a sibling of the top element (could easily happen ith a `\n` line feed added by an editor). This is fixed by filtering on IElement with `elements()`. A `parse` instance has been added as it is not necessary to create a new `ElementParser` each time that we want to parse something.

You'll find install instructions in documentation (see doc/installation.rst).

If you need help, come the SàT official XMPP room: sat@chat.jabberfr.org or join us with a web browser via https://chat.jabberfr.org/converse.js/sat@chat.jabberfr.org