Implement basic product, category and brand API

[erikhofer]

• product
  • [x] get a specific product by
  • [x] create product
• category
  • [x] create category
  • [ ] get all categories
  • [ ] get all products in a specific category
• brand
  • [ ] get all brands
  • [ ] get all products from a specific brand

[erikhofer]

Subtasks:

• [x] use serials (see #14) instead of IDs. For this, rebase branch to master
• [x] sample data (especially for category and brand)
• [x] create product / category should return something, at least the serial of the created entity
• [x] validation (when creating entities)
• [x] authorization
• [x] error handling
• [x] tests (at least happy path to make sure that basic functionality doesn't break)