[FEATURE] show unavailable messages of contacts

This feature allows to view the unavailable messages of the contacts
which can be set by almost any IM application (but not in Saros
currently :P).

Change-Id: I0dbe4d40fdffce83c237f8d1a6c007b63a65d034
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1327
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>
Tested-by: Jenkins CI