[fix] corrected multipart MailOrder parsing: skip first part and parse 'content-*' headers properly
[licence] updated copyright years and added missing headers
[evol] translation rework: new mechanism to handle translated/untranslated strings, make use of translation parameters, moved I18n class, translations update
[fix] correct silly mistake
[evol] commands management rework #2: moved command parsing out of the Mail class and created new classes to handle it