Merge: DOM XML

Hi,

As Nit programmers, we might have to parse XML sometimes for one use or another.

We already had a SAX parser in library for the cases when one wanted to parse a long XML document in a memory-conscious way, often at the expense of the simplicity of the end-program.

This PR adds a very simple DOM parser that will need to be improved and robustified later, but who right now does the basics and should parse valid xml documents.

There is still a long way to get a full spec-compliant XML parser, but this is one base.

Pull-Request: #1448
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>