[INTERNAL] changed implementing interface of CreateArchiveTask class

It now implements the internal IWorkspaceRunnable interface of the Saros
Core file system instead of the Eclipse IWorkspaceRunnable interface.

Change-Id: Ibd5d8bb518b1c1c0298d00263300f1be80c63d8c
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1803
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>