Make attachment have a xml_subtree
Make attachment have a xml_subtree

...much like item classes do.

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