[INTERNAL] add SarosContextFactory, SarosIntellijContextFactory

Change-Id: Ia740fe0022493a7974a7f1efea4f777d99b94666
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1747
Tested-by: Jenkins CI
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>