Replace Pipework with Docker 0.6.5+ container linking

Instead of using Pipework to explicitly assign IP addresses to containers, this pull request contains changes to leverage Docker 0.6.5+ [container linking](http://docs.docker.io/en/latest/use/working_with_links_names/).

The first node in the cluster acts as a seed node for others. As the second node comes up, it detects whether it is a seed or not. If it isn't, it sends a cluster join request to the seed. Once all of the nodes are connected, a cluster planning and committing phase is required to make the cluster live.

(All of the above is handled by `make start-cluster`.)

Resolves:

- https://github.com/hectcastro/docker-riak/issues/7
- https://github.com/hectcastro/docker-riak/issues/6