Should gracefully handle a pool of nodes as well

[ixmatus]

On creation of a pool, it would be nice to feed it a list of hosts; use case here would be if a connection slows - M/R throws a 500, or timeouts start occurring from heavy load on a specific node, riakpool can failover to a less busy node.

I'm thinking of adding in this code but it also requires being able to query a riaknode for its "load" which isn't currently possible. I may contribute a change that offers that through the Riak PBC API and then work on adding this functionality to RiakPool.

Making a ticket here so I don't forget about this project, as it would make more sense than a round-robin proxy sitting in front of the nodes because this would enable the riakpool manager to know the state of each cluster member and serve execution requests based on which member is most available. A powerful feature IMHO and very doable.