Database Postgresql in carbonio

@keita: It's quite difficult to answer in short... PostgreSQL is the main database used by Carbonio and its modules and is used also by other services that are not run by Carbonio directly, but are required for Carbonio to work properly. We might add in the future to the documentation some more detail about the architecture of Carbonio and the (inter)relation between the various components, although this would take quite a lot of time (it is low priority at the moment)