Cluster object for registries

AutoConsul::Cluster
- Was a module, but now it's a class
- Has an "agents" registry
- Has a "servers" registry
- Based on an initial uri
Runner methods for invoking 'consul agent'

AutoConsul::Runner module:
- run_agent! singleton method launches an agent and joins a cluster
  by picking a member from the agents registry
- run_server! singleton method launches in server mode
- if there's a server in the registry, joins with it
- if not, uses -bootstrap to initialize the raft cluster.