Support Ganglia v30x and v31x protocols.

- Opens an UDP port (8649) to receive Ganglia v30x and v31x protocol messages.
- Decodes Ganglia messages using gmetric4j and xdr.
- Inserts just numeric metrics to OpenTSDB.
- Added ganglia version, hostname, ip address, and optionally cluster name to
  the metric as tags.
- Ignored units.
- Ignored string metrics.
- Collected Ganglia RPC statistics.
- Ganglia support is disabled by default.