Implement relational model collection

[cebe]

• [ ] relational operations like link() and unlink() sync()
  • https://github.com/yiisoft/yii2/pull/12304#issuecomment-242339800
  • https://github.com/yiisoft/yii2/issues/10806#issuecomment-242346294

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• [ ] addToRelation() by checking if query is a relation
  • https://github.com/yiisoft/yii2/issues/10806#issuecomment-241505294

[cebe]

http://www.yiiframework.com/extension/activerecord-relation-behavior