[Proposal] Implement save() method on the BelongsTo relationship

I feel it would be reasonable to have a method to persist the relationship from the side of the BelongsTo relationship. I do know that this is available through the side of `HasOneOrMany::save()` however this limits one from persisting a mixed batch of models against another mixed batch (one would be forced to juggle `instanceof` statements to sniff which type of relationship was being given). A generic API for persisting a hasMany/belongsTo relationship seems like something Eloquent should support.

Example:
    
```php
    class Country {
        public function users()
        {
            return $this->hasMany('User');
        }
    }

    class User {
        public function country()
        {
            return $this->belongsTo('Country');
        }
    }

    $user = User::find(123);
    $country = Country::find(1);
    
    // HasMany::save(Model $model)
    $country->users()->save($user);
    
    // BelongsTo::save(Model $model)
    $user->country()->save($country);
```