Refactor-out mailbox XML node creation
Refactor-out mailbox XML node creation

Code will be used in different places. Makes interface more
understandable I think:

    std::string to_xml(const char* xmlns) const
    xml_node<>& to_xml_element(xml_node<>& parent) const
    static mailbox from_xml_element(const xml_node<>& elem)

m-Schlitzer/ews-cpp,otris/ews-cpp,otris/ews-cpp,bkircher/ews-cpp,m-Schlitzer/ews-cpp,m-Schlitzer/ews-cpp,bkircher/ews-cpp,otris/ews-cpp,otris/ews-cpp,bkircher/ews-cpp