[FEATURE] Changed structure extensions to serializable data arrays in structure

__Tasks:__

- [x] test coverage
- [ ] gather feedback for my changes
- [ ] submit changes to the documentation


__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | no => https://github.com/sulu-cmf/SuluTestBundle/pull/19
| Fixed tickets | https://github.com/sulu-cmf/SuluContentBundle/issues/159
| BC Breaks     | changed interface to get extension data (described in Issue)
| Doc           | TODO
@wachterjohannes Wouldn't you have let this method names the same? At least for me it was just meant to be easier accessible in the twig template, since we have no auto complete there. (And to me it looks like the other method is still called `hasExtension` ;))