Include scope

This allows the usage of all the filter params (where, order, fields, include, ...) on included relations.

The syntax is as follows  `{ relation: '<relation>', scope: { ... } }`:

```
Category.find({
  include: {relation: 'posts', scope:{
    fields: ['title'], include: ['author'],
    order: 'title DESC'
  }}
}, function(err, categories) {
  console.log(categories)
});

User.find({
  include: {posts: {relation: 'categories', scope:{
    order: 'seq DESC'
  }}}
}, function(err, users) {
  console.log(users)
});
```