[FEATURE] Take first steps towards Saros Server


Register server feature in XMPP, create entry in Preference Store to
activate a server.
Add package extensions to command a server to start an empty session
with yourself or invite you to a running session.

Changes are not yet active (see saros.properties).

Change-Id: I9c99bc4108d96ee036f822483b1acabbd8963bc5
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1279
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>
Reviewed-by: Arndt Lasarzik <alasarzik@gmail.com>
Reviewed-by: Nils Bussas <nils.bussas@gmail.com>