Add support for ability to start and stop Swarm via command `service swarm on/off`.
Fix issue with swarm not starting due to missing web3 object.
`service swarm on` - starts a swarm node if not already started. Shows an error if the node is already starting or started.
`service swarm off` - kills the running swarm node as long as Embark has started the swarm process. If the swarm process was started externally, an error is shown.