get number of unread messages

add functions to get the number of unread messages. Clients get the
number of unread messages in a conversation when getting
ConversationInfo.

additional test coming with : testCount, count the number of
occurrences in the database matching a value.

Change-Id: Ie7db0b7f288bfe82b64518dd1e9790837c2a2087
Reviewed-by: Sébastien Blin <sebastien.blin@savoirfairelinux.com>