Fix for adding children with << operator when order_is_numeric?

Added tests for using << operator to add children when order_is_numeric? is true.  (Proved bug)

Fix for Issue #84 where the order_value was left as nil when order_is_numeric? is true.

Fix for bad SQL statements when parent_id is nil (tree roots).  Caused ordering problems for roots because root nodes wouldn't be matched by reorder_with_parent_id for PostgreSQL or MySQL.