Add 'otherKey' methods to BelongsTo

Useful for building dynamic relation queries or joins.

My use case, which would break if I change the relation otherKey:

```php
public function getAdmins($company = null)
{
  $relation = $this->model->company();
  $foreignKey = $relation->getForeignKey();

  if ($company !== null) {
    // $otherKey = $relation->getOtherKey();
    $otherKey = $company->getKeyName();
    $include = $company->getAttribute($otherKey);
  } else {
    $include = null;
  }

  $query = $this->model->newQuery()
    ->where(function($query) use($foreignKey, $include) {
      $query->whereNull($foreignKey);
      if ($include !== null) {
        $query->orWhere($foreignKey, '=', $include);
      }
    });

  return $query->get();
}
```