Xml schema testing

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #5
| License       | MIT
| Doc PR        | symfony-cmf/symfony-cmf-docs#335

This PR is the continuation of #9 

Instead of using helpers, I used annotations. This results in better error messages. It is also fully tested.