20 instead of 6 images? (AssertionError)

Can someone please tell me, why 6 png images are expected by this test? Using `ffmpeg` is broken on my maschine because `ffmpeg` is deprecated and I only get two `data` events but if I change using `ffmpeg` to `avconv`, i get 20 png images (3 broken, 17 with a printer and a cup).

See #60 to find out why I want to use `avconv` instead of `ffmpeg`.
### Environment

OS: Ubuntu 12.04
Node: 0.8.25
NPM: 1.2.30
node-ar-drone: 031d2e38
### Using ffmpeg (deprecated)

```
$ npm test

> ar-drone@0.2.1 test /home/jan/workspace/node-ar-drone
> node test/run.js

[0:00:01 0 14/17 82.4% node test/integration/video/PngEncoder/test-sample-encode.js]

assert.js:102
throw new assert.AssertionError({

AssertionError: 2 == 6
at process. (/home/jan/workspace/node-ar-drone/test/integration/video/PngEncoder/test-sample-encode.js:23:10)
at process.EventEmitter.emit (events.js:96:17)

[0:00:01 1 16/17 100.0% node test/integration/video/TcpVideoStream/test-connection-timeout.js]
```
### Using avconv

Diff: https://gist.github.com/janpieper/7081811 (prototype)

```
$ npm test

> ar-drone@0.2.1 test /home/jan/workspace/node-ar-drone
> node test/run.js

[0:00:01 0 14/17 82.4% node test/integration/video/PngEncoder/test-sample-encode.js]

assert.js:102
throw new assert.AssertionError({
^
AssertionError: 20 == 6
at process. (/home/jan/workspace/node-ar-drone/test/integration/video/PngEncoder/test-sample-encode.js:23:10)
at process.EventEmitter.emit (events.js:96:17)

[0:00:01 1 16/17 100.0% node test/integration/video/TcpVideoStream/test-normal-connection.js]
```