Add support for SolrCloud client and a subset of Collections API

This pull requests exposes the CloudSolrServer and a limited subset of the SolrCloud Collections API. 

Once this pull request has been merged, users should be able to instantiate a cloud client, create/delete collections and fetch a list of replicas and filter them by various attributes such as state, node and leader.

I've added documentation about the usage of this API to the README.md file too.