Support fields of type `relation`

[joelmukuthu]

Something like:

User.fields = { 
  id: 'integer',
  groups: {
    type: 'relation',
    // configure the relation here somehow
  }
};
Group.fields = {
  id: 'integer',
  users: {
    type: 'relation',
    // configure the relation here somehow
  }
};
Membership.fields = {
  userId: {
    type: 'integer',
    references: User.fields.id
  },
  groupId: {
    type: 'integer',
    references: Group.fields.id
  }
};

• Support fetching related models as fields:

User.fetch({ fields: ['id', 'groups']  })

• Supporting fetching related models:

new User({ id: 1 }).groups.fetch();

• Supporting inserting related models:

new User({ id: 1 }).groups.insert({ id: 1 });

• Supporting updating related models:

new User({ id: 1 }).groups.update({ foo: 'bar' });

• Supporting deleting related models:

new User({ id: 1 }).groups.delete();