Add stubs and class diagram

add stubs for :
 - Database class.
 - ConversationModel class and Conversation namespace.
 - NewCallModel class and NewCall namespace.
 - ContactModel class and Contact namespace.
 - Message namespace.
 - NewAccountModel.
 - CallbacksHandler, will be used to bind the callbacks from the daemon
   and will emit signals for lrc and client.
 - Lrc class, this class is the root class of all models. Instating this
   class will build interfaces between the daemon, client and the
   database.
 - all these class are nested in the lrc namespace.
 - all publicly exposed API have been put into lrc::api namespace
 - add diagrams in doc folder

Change-Id: Ia06f874dd1baebb086edd570e52d8d61d73b905b
Reviewed-by: Guillaume Roguez <guillaume.roguez@savoirfairelinux.com>
Signed-off-by: Guillaume Roguez <guillaume.roguez@savoirfairelinux.com>