Added support for custom level mapping.

User can provide a levelMap object in options object passed to the contructor(exported as Graylog2). This will be used to map winston levels to graylog2 levels, instead of the default mapper. If a level has no mapping then it will be mapped to level 'info' (same as before).
not sure this would work without hacking around node-graylog2's [_log](https://github.com/Wizcorp/node-graylog2/blob/master/graylog.js#L109) method.

```javascript
 this.graylog2._log('', msg, meta, (new Date()).getTime() / 1000)this.getMessageLevel(level));
```