[VarDumper] Use Unicode Control Pictures

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

I wasn't satisfied with how control chars where dumped, until I found the [Unicode Control Pictures](http://www.unicode.org/charts/PDF/U2400.pdf).
This PR takes advantage of them:

![capture du 2014-11-04 10 52 15](https://cloud.githubusercontent.com/assets/243674/4897678/752fac20-6408-11e4-9186-7b788c9a88bb.png)