Possible memory leak warning

Heyho,

I just installed markdown-pdf via `npm install markdown-pdf -g` on my laptop, and when using it, it tells me, there is a possible memory leak:

```
$ markdown-pdf myMarkdown.md
(node) warning: possible EventEmitter memory leak detected. 2 end listeners added. Use emitter.setMaxListeners() to increase limit.
Trace
at SeriesStream.addListener (events.js:252:17)
at SeriesStream.Readable.on (_stream_readable.js:673:33)
at SeriesStream.on (...\npm\node_modules\markdown-pdf\node_modules\series-stream\index.js:12:38)
at SeriesStream.once (events.js:278:8)
at SeriesStream.Readable.pipe (_stream_readable.js:486:9)
at SeriesStream.pipe (...\npm\node_modules\markdown-pdf\node_modules\series-stream\index.js:22:30)
at ...\npm\node_modules\markdown-pdf\node_modules\stream-from-to\index.js:101:25
at ...\npm\node_modules\markdown-pdf\node_modules\mkdirp\index.js:47:26
at FSReqWrap.oncomplete (fs.js:82:15)
```

Using windows7, node 5.10.1

Output looks fine and as expected.