[REFACTOR] moved ObservableValue classes to Saros Core

Change-Id: Ib1222868216e85bf10a0123b3c62859aabc4200f
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1428
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>