Add more styling

Related to #131

I added some more styling and removed dash/plus signs as suggested: I think it's more consistent with the Symfony Console where error messages are displayed in borderless blocks.

Here is what it looks like:

![symfony_check](https://cloud.githubusercontent.com/assets/327237/3487117/bd50fa3e-0465-11e4-8fcc-46b29fb9563c.png)