[WIP] [FEATURE] Added snippet repository and REST API

Added a rest API:

````
 get_snippets GET    ANY    ANY  /api/snippets.{_format}        
 get_snippet  GET    ANY    ANY  /api/snippets/{uuid}.{_format} 
 post_snippet POST   ANY    ANY  /api/snippets.{_format}        
 put_snippet  PUT    ANY    ANY  /api/snippets/{uuid}.{_format}
````

I had a bit of a worry about if we should reuse the existing `NodeController` REST interface in `ContentBundle`. It does exactly the same thing, we only need to add a `type` parameter.

But in the end I thought the API is clearer with `/api/snippets`. ( We do however need to share logic between the two controllers.

## Depends:

- ContentBundle (features/pages-refactor) https://github.com/sulu-cmf/sulu/pull/277

## Notes:
- **WIP**: Missing docblocks etc.
- **Security**: I guess everything under `/api` is secutre right?