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 should use a service check instead of a gauge here to report the status of fluentd. You can take a look at one PR adding service checks, like this one for instance #1163.