add bin to package.json

add bin to package.json, so when I install the module, the `karma` will be added to node_modules/.bin

I can add the script to my project's package.json

```js
"script": {
  "test": "karma .........."
}
```

for now, I need
```js
"script": {
  "test": "node_modules/karma/bin/karam .........."
}
```