Added the class DbInitializer, which creates the necessary tables
from a xml based description which is part of the binary (Qt rcc).

So when akonadiserver is started the first time, it creates all tables
automatically now. Changes to the tables structure have to be done in

server/src/storage/akonadidb.xml

In a later version DbInitializer should also fill the tables with basic
data (mimetypes, flags etc.)