Node#only_node? returns true if only one node is known
Fix Cluster.create! checks
Can't forget node that has slots
Add Cluster#each method
Ruster::Cluster.create! returns a new instance of Ruster::Cluster
Fix typo
Add Ruster::Cluster#reshard

Note that this method does an even resharding from all the source
nodes, and it's not accepting any parameters.
Refactor bin/ruster to use Ruster classes