test: add hasAndBelongsToMany integration test

This pull request adds two integration tests for strongloop/loopback-datasource-juggler#85

- it allows to find related objects via id filter

  `GET /api/products?filter[where][categoryId]=CAT-ID`

- it fills related objects requested by "include" in find

  `GET /api/categories/findOne?filter[where][id]=CAT-ID&filter[include]=products`

What is not covered yet:

- it fills related objects requested by "include" in findById
 
  `GET /api/categories/CAT-ID?include=products`

/to @raymondfeng please review.
/cc @ritch @pandaiolo

Related: strongloop/loopback-angular#26