MailPlugin: skeletal implementation.
DAO: small changes in the API. Add MailDAO. Move core actor references to BasePlugin.
MailPlugin: initial database version and DAO.
MailPlugin: small fixes, enable plugin.
JabberProtocol: improve message send status.
MucMessageHandler: improve participant handling in some weird cases.
MailPlugin: put messages to the queue only when the queue is not full.
MailPlugin: final tuning.
Merge branch 'origin/mail'. Close #43.