[REFACTOR] Delete SPathDataObject

This patch introduces an SPathConverter which allows XStream to handle
SPath objects directly instead of using the intermediate format of an
SPathDataObject.

Change-Id: I3731c9c2313b4e2c7d8e95b4b625f39a09f95c35
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1522
Tested-by: Jenkins CI
Reviewed-by: Arndt Lasarzik <alasarzik@gmail.com>
Reviewed-by: Damla Durmaz <ddurmaz88@gmail.com>
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>