[REFACTOR] moved escape/unescape methods from Utils class

They were only used by the XStream converters. Moved them to a simple
URLCodec wrapper class.

Change-Id: I1f76160c08406c0447f2fc384460a52455999aa4
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1318
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>