Create API bundle

 * Removes the generic Middleware bundle and creates an API bundle in its place
 * Allows for development of other bundles
 * Allows for non-bundle code

Merges into `feature/event-sourcing` master PR.
oh, it's fine indeed, as said it's just a feeling... however wouldn't that be the same as: 

```json
{
    "psr-4": {
        "Surfnet\\": "src\\Surfnet",
    }
}
```

as that covers both cases? It would require a relocation of files though (`src\ApiBundle` vs `src\Surfnet\StepupMiddleware\ApiBundle`), hence the reason I asked :wink:

In any case, I'll leave making a decision to you :tongue: