Add check for fluentd

Add AgentCheck for [Fluentd](http://www.fluentd.org/). This check enables us to create a dashboard like this:

![fluentd___datadog](https://cloud.githubusercontent.com/assets/290039/3965005/e7c89da0-2790-11e4-84f4-4c986e033c98.png)

You can set that as a class constant because it's not to be changed:

```
class Fluentd:
    SERVICE_CHECK_NAME = 'fluentd.is_ok'
```