Undefined event

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Hi there!
thank you for this project, it is exactly what I was looking for. However I have an issue at my first try:

```
~/project/node_modules/input-event/lib/keyboard.js:18
if(InputEvent.EVENT_TYPES.EV_KEY == ev.type){
^

TypeError: Cannot read property 'type' of undefined
```

My implementation is quite simple:

``` js
'use strict';

var InputEvent = require('input-event');

var mice = new InputEvent('/dev/input/mice');
var mouse = new InputEvent.Mouse(mice);

mouse.on('keyup' , console.log.bind(null, 'mice:'));
mouse.on('keydown' , console.log.bind(null, 'mice:'));
mouse.on('keypress', console.log.bind(null, 'mice:'));
```

I'm running it on an Ubuntu 16.04, reproducing the error with both node **v0.10.45** and node **v6.3.0**. Do you have any clue?

Start by reading lib/keyboard.js around line 18 and reproduce the failure using /dev/input/mice on the reported Node and Ubuntu versions. Trace why the event value is undefined, then verify that mouse event listeners no longer crash when processing device input.

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