[WIP] Refactoring for Snippets

- Make Structure a super type
- "Page" will take role of current Structure
- Snippet will be subclass of Structure

## Depends

- [x] https://github.com/sulu-cmf/SuluTestBundle/pull/25 
- [ ] https://github.com/sulu-cmf/SuluTestBundle/pull/26

## Todo

- <strike> Sub-class the template reader. </strike> (handled in separate PR)
- <strike> Enforce the `title` property on Snippet templates.</strike>
- [x] Webspace should not be required by the API
- [ ] Either refactor the ContentMapper test or not. but in anycase, add Snippet tests^
- [x] Handle "not found" snippets when retrieving multiple snippets by ID (do not 500) - (fixed in SnippetBundle)

^ The snippet mapping is already well tested by the SnippetBundle, but we should add tests here too.
This line should actually look like:

```php
$data = array_merge(
    array(
        'webspaces' => array(),
        'snippets' => array(),
    ),
    Yaml::parse(file_get_contents($fixturePath . '/' . $file))
);
```

IMHO same for the `sprintf`-calls I have seen here, but we already had this discussion ;-) Would prefer to be consistent here.