Implement cluster metadata REST endpoints

[detro]

* GET /brokers     - Cluster meta and list of Brokers
* GET /topics      - List of Topics
* GET /topics/{t}  - List of Partitions for Topic t
* GET /groups      - List of Consumer Groups
* GET /groups/{g}  - List of Members for Consumer group g

The output should be in JSON format, so to play nicely with pretty much any tool that a user might want to assemble against it.