[API] [1.0] Changed the namespace for `cluster.nodes_*` APIs to `nodes`

Due to changes in Elasticsearch 1.0 API, the client "namespaces" have been
changed to reflect the change in URLs: `_cluster/nodes` => `_nodes`

Thus, instead of:

    client.cluster.nodes_info

the new API is:

    client.nodes.info

Related:

* elasticsearch/elasticsearch#4055
* elasticsearch/elasticsearch#4336
* elasticsearch/elasticsearch#4769