[WIP][DX] Better exception page

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   |
| Fixed tickets | 
| License       | MIT
| Doc PR        |

* [x] add twig files into stack trace
* [x] add syntax highlighters for PHP and Twig with symfony.com's color scheme
* [ ] use VarDumper for function arguments (maybe in another PR)
* [ ] add tests

**Before:**
![before](https://cloud.githubusercontent.com/assets/288535/4853483/61ef59a6-6086-11e4-800a-6771f0c0f32d.png)

**After:**
![after](https://cloud.githubusercontent.com/assets/288535/4853487/74d5b5ec-6086-11e4-9708-cafcbacf3d49.png)
the naming is a bit weird here:

- your ExceptionProcessor is taking an Exception and flattening it into a FlattenException
- your ExceptionFlattenerInterface is processing a FlattenException to alter the way it was flattened.

The names and the responsibilities look swapped