Merge: xml: Introduce SAXophoNit, a SAX processor in Nit.

Merge: xml: Introduce SAXophoNit, a SAX processor in Nit.

This is a work in progress for a SAX processor in Nit.

Here is a sumary of what is (and what is not) supported at this point:
- [X] Content
- [X] XML declaration (parse it)
- [X] Acceptation of any XML document that Doxygen generates
- [ ] `DOCTYPE` declaration (or ignore it)
- [ ] XML declaration (do something with the settings)
- [x] Namespaces in XML
- [x] SAX mandatory feature flags
- [ ] XML 1.1
- [ ] Ignorable white space
- [ ] User-defined entities
- [ ] Encodings
- [ ] URIs
- [x] Test suite

Pull-Request: #802
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>