Extend adapter to support automatic includes

Closes #3325
- Add Roles model and add hasMany roles to User model.
- Add EmbeddedRelationAdapter that will automatically include
  hasMany relations in calls to the API.
- UserAdapter and PostAdapter now extend EmbeddedRelationAdapter
  and all explicit includes from store.find() have been removed.
Merge pull request #3346 from jaswilli/issue-3325

Extend adapter to support automatic includes