Add nxlog parser

See issue #9

I switched `SOURCE_HOSTNAME` to `SOURCE-HOSTNAME` to respect old school DNS conventions disallowing underscores (which logstash grok patterns [observe](https://github.com/elasticsearch/logstash/blob/master/patterns/grok-patterns#L27)).
@dpb587 - I've added some tests to check that the wrapped log event is processed correctly.  In the process I fixed a bug with the `shipper[source_params]` matcher being too greedy