Implement db.Model.BelongsTo(anotherModel)

Checks whether a model's key uses another model's key as its parent.

```
type Account struct {
   Name string `db:"id"` 
}

type Post struct {
  Title string
}

// ommit models initialization

account := &Account{}
err := datastore.Load(account)

post := Post{"A new awesome post"}
// Sets account as the post's parent
post.BelongsTo(account)
```
