added the SensioLabs security checker

Usage:

```
./app/console security:check
```

![screenshot](https://cloud.githubusercontent.com/assets/47313/3634555/80738bd2-0f36-11e4-92d3-890adfb89882.png)

![screenshot](https://cloud.githubusercontent.com/assets/47313/3634556/90f38c6e-0f36-11e4-82d6-cdf5bdfbd1bd.png)