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, We spoke yesterday of having a `@filters` hash to contain information about which filters were used.

It occurs to me that we actually already store that data, but kinda mashed together into the `tags` field, where we put both information about successes - eg, `syslog_standard`, `nginx`, and failures - eg `_grokparsefailure-nxlog_standard`

I'm thinking we should categorise this existing information something like this: 

```
@parser: { 
   success: ['syslog_standard', 'nxlog_standard'], 
   failures: ['_grokparsefailure-nginx_combined']
}
```