Add an XmppParser, and Stanza classes

Generate events on stanza and stream open/close.
Create Stanza and serialize them.

Note: XML namespaces are not handled yet.

src/xmpp/xmpp_stanza.cpp

@@ -0,0 +1,127 @@
+#include <xmpp/xmpp_stanza.hpp>
+
+#include <iostream>
+
+XmlNode::XmlNode(const std::string& name, XmlNode* parent):
+  name(name),
+  parent(parent),
+  closed(false)
+{
+}
+
+XmlNode::XmlNode(const std::string& name):
+  XmlNode(name, nullptr)
+{
+}
+
+XmlNode::~XmlNode()
+{
+  this->delete_all_children();
+}
+
+void XmlNode::delete_all_children()
+{
+  for (auto& child: this->children)
+    {
+      delete child;
+    }
+  this->children.clear();
+}
+
+void XmlNode::set_attribute(const std::string& name, const std::string& value)
+{
+  this->attributes[name] = value;
+}
+
+void XmlNode::set_tail(const std::string& data)
+{
+  this->tail = data;
+}
+
+void XmlNode::set_inner(const std::string& data)
+{
+  this->inner = data;
+}
+
+void XmlNode::add_child(XmlNode* child)
+{
+  this->children.push_back(child);
+}
+
+void XmlNode::add_child(XmlNode&& child)
+{
+  XmlNode* new_node = new XmlNode(std::move(child));
+  this->add_child(new_node);
+}
+
+XmlNode* XmlNode::get_last_child() const
+{
+  return this->children.back();
+}
+
+void XmlNode::close()
+{
+  if (this->closed)
+    throw std::runtime_error("Closing an already closed XmlNode");
+  this->closed = true;
+}
+
+XmlNode* XmlNode::get_parent() const
+{
+  return this->parent;
+}
+
+const std::string& XmlNode::get_name() const
+{
+  return this->name;
+}
+
+std::string XmlNode::to_string() const
+{
+  std::string res("<");
+  res += this->name;
+  for (const auto& it: this->attributes)
+    res += " " + it.first + "='" + it.second + "'";
+  if (this->closed && !this->has_children() && this->inner.empty())
+    res += "/>";
+  else
+    {
+      res += ">" + this->inner;
+      for (const auto& child: this->children)
+        res += child->to_string();
+      if (this->closed)
+        {
+          res += "</" + this->name + ">";
+        }
+    }
+  res += this->tail;
+  return res;
+}
+
+void XmlNode::display() const
+{
+  std::cout << this->to_string() << std::endl;
+}
+
+bool XmlNode::has_children() const
+{
+  return !this->children.empty();
+}
+
+const std::string& XmlNode::operator[](const std::string& name) const
+{
+  try
+    {
+      const auto& value = this->attributes.at(name);
+      return value;
+    }
+  catch (const std::out_of_range& e)
+    {
+      throw AttributeNotFound();
+    }
+}
+
+std::string& XmlNode::operator[](const std::string& name)
+{
+  return this->attributes[name];
+}