Flowdock notifications

This PR implements [Flowdock](https://www.flowdock.com/) notification plugin. Flowdock notifications can be triggered with the following options:
```
notify:
  flowdock:
    token: "{Flow API Token}"
    source: "CI"
    tags: "drone, master"
    on_started: true
    on_success: true
    on_failure: true
```

Notification looks like this in the Flowdock's team inbox:
![screen shot 2014-08-26 at 10 25 24](https://cloud.githubusercontent.com/assets/455844/4095918/c8909bc2-2fba-11e4-8bd3-14ee99a24dae.png)

Plugin has a dependency to "[github.com/stvp/flowdock](https://github.com/stvp/flowdock)"