Add optional support for load balancing across multiple Riak nodes

[agemooij]

The java client currently supports builtin optional load balancing using something called a "cluster client". See:

https://github.com/basho/riak-java-client/wiki/ClientFactory#http-or-pb-cluster-client

In my own projects I've used a "real" load balancer in front of our Riak clusters so I never needed this but it makes sense as an optional feature.

Requested by @tjweir, see:

https://twitter.com/tylerweir/status/313657579362463744

[tjweir]

Just as an update from my perspective. My production environment uses a load balancer in front of our Riak cluster.

For development, I use the cluster client. So, I would put this priority at low.

Thanks!