Add custom log method

#### Changes

* `_prepJson` accepts the level as a string
* added .log method, where the level is passed as a parameter

#### Use case

I'm writing a [winston](https://github.com/flatiron/winston) graylog2 transport, and I'm using graygelf to talk to graylog. Instead of picking the appropriate graygelf method `.emerg`, `.error`, etc, it would be cleaner if graygelf had a `.log` method which also accepts the level.