Do not use ':' as a namespace separator with expat

Instead use \1, and build our own nodes by explicitely separating the
namespace and the node name.

src/xmpp/xmpp_parser.hpp

@@ -18,9 +18,9 @@
  * stanza is reasonnably short.
  *
  * The element names generated by expat contain the namespace of the
- * element, a colon (':') and then the actual name of the element.  To get
+ * element, a \1 separator and then the actual name of the element.  To get
  * an element "x" with a namespace of "http://jabber.org/protocol/muc", you
- * just look for an XmlNode named "http://jabber.org/protocol/muc:x"
+ * just look for an XmlNode named "http://jabber.org/protocol/muc\1x"
  *
  * TODO: enforce the size-limit for the stanza (limit the number of childs
  * it can contain). For example forbid the parser going further than level