Adding JsonLogster parser.

This can be used to parse log files in which there is one json object per line.
Each json object will be flattened into metrics keyed by the concatenated
keys in the original json object.

I'm writing this for the Wikimedia Foundation.  We are starting to use Kafka to collect web access logs from varnish, using a varnishkafka module.  varnishkafka outputs periodic JSON stats to a file, and I'd like to use Logster to send them to Ganglia or whatever.

Here's an example subclass of JsonLogster:
https://gist.github.com/ottomata/7663920