JSON output format for clusterdumper

[Telvis Calhoun Jr.]

I've created a Jira - https://issues.apache.org/jira/browse/MAHOUT-1343.
The jira has example output and a patch. I needed JSON output from
clusterdump so I added a JSON output format option, a JsonClusterWriter
class and unittest.

Thoughts? Concerns?

My gh fork is here:
https://github.com/telvis07/mahout/tree/mahout_clusterdump_json.

Thanks.
- Telvis Calhoun