Improved app/check.php output

```
| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | n/a
| Fixed tickets | none
| License       | MIT
| Doc PR        | none
```

This pull request propose a more readable output for app/check.php

For the purpose of the screenshots, I enabled all "help text", (those preceded with a pipe, actually displayed only on warning/error)

## Before:
![app_check_before](https://cloud.githubusercontent.com/assets/19408/2808730/a91449e6-cd41-11e3-86ca-f0a5c7a14ec8.png)

## After:
![app_check_after](https://cloud.githubusercontent.com/assets/19408/2808731/a9148956-cd41-11e3-8469-fb35c1424e2e.png)