Add implementation for FindFolder
Add implementation for FindFolder

The new find_folder(const folder_id&) function allows querying
of subfolders in a specific folder.
This includes find_folder_response_message to parse the response.

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