Add, Remove, and query Relations

We've added two method to Object class in order to manage relations.
With this changes we can add and remove relations with the following syntax:
object.addRelation(key, className, objectsId)
or 
object.removeRelation(key, className, objectsId)

We also can perform relational queries with Queryset using the key "relatedTo". ie.
comments = Comment.Query.filter(object__relatedTo={"object":{"__type":"Pointer","className":"Product","objectId":product.objectId},"key":"comments"})