[API] add getID to ISarosSession interface

Especially for the components of the session container this method is
much more useful than to obtain the id via the session id observable.

Change-Id: I756c275915c795ee4ebf86c8ef74d10b6295f121
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1134
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>