[REFACTOR] moved XStream util classes into Saros Core

Change-Id: I2da73ce6dab260231d41db24ff016e08e2e78c9f
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1411
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>