[API] Added core.net.business package with CancelHandlers

Cleaned up InvitationHandler.java to not use SarosSessionObservable.

Change-Id: Idf6643885e402cdce1d511628d40d9197c68f4bc
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1846
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>
Reviewed-by: Holger Schmeisky <google@schmeisky.com>