[FEATURE] Added categories and tags to excerpt tab

Additionally added except.xml to generate excerpt form with the form generator

__Tasks:__

- [x] test coverage
- [ ] gather feedback for my changes

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | no => https://github.com/sulu-cmf/sulu/pull/194
| Fixed tickets | https://github.com/sulu-cmf/SuluContentBundle/issues/178
| BC Breaks     | tags or categories which were used in xml-template should be removed and the data should be stored in excerpt tag. to fix the bc break take the `content.ext.excerpt.categories` or `content.ext.excerpt.tags`
| Doc           | none