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 probably want to raise the exception here instead of the string message:
```
>>> raise "x"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: exceptions must be old-style classes or derived from BaseException, not str
```
Also if you want to detect this during tests, it's relatively easy to add another instance in the test case with a dummy URL that will throw a critical service check :wink: