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)).
I always question how to handle this situation. Optimally I want a negated character set like "\\]" to allow for escaped end brackets in the first part. But I haven't gotten that quickly working to be able to make it effective.

Your change is good - it seems more likely that message would have a bracket than the NXLOG part - thanks.