Fixed behavior when *_destroy callbacks are critical.

For example:
```ruby
class Category < ActiveRecord::Base
  acts_as_ordered_tree

  before_destroy :crit
  def crit
    # something very critical
  end
end

root = Category.create
child = root.children.create

root.destroy # callback called only for root, children deleted silently
```

Now <code>destroy</code> method called for every descendant of destroyed node.

lib/acts_as_ordered_tree.rb

@@ -33,7 +33,8 @@ def acts_as_ordered_tree(options = {})
       :class_name    => name,
       :foreign_key   => options[:parent_column],
       :order         => options[:position_column],
-      :inverse_of    => (:parent unless options[:polymorphic])
+      :inverse_of    => (:parent unless options[:polymorphic]),
+      :dependent     => :destroy
     }
 
     [:before_add, :after_add, :before_remove, :after_remove].each do |callback|
@@ -83,7 +84,7 @@ def acts_as_ordered_tree(options = {})
     after_save "move_to_child_with_index(parent, #{position_column})",
                :if => "#{position_column} && (#{position_column}_changed? || #{parent_column}_changed?)"
 
-    before_destroy :destroy_descendants
+    before_destroy :flush_descendants
     after_destroy "decrement_lower_positions(#{parent_column}_was, #{position_column}_was)", :if => position_column
 
     # setup validations

lib/acts_as_ordered_tree/instance_methods.rb

@@ -329,9 +329,7 @@ def set_scope! #:nodoc:
       end
     end
 
-    def destroy_descendants #:nodoc:
-      descendants.delete_all
-      # flush memoization
+    def flush_descendants #:nodoc:
       @self_and_descendants = nil
     end
 

lib/acts_as_ordered_tree/version.rb

@@ -1,3 +1,3 @@
 module ActsAsOrderedTree
-  VERSION = "1.0.3"
+  VERSION = "1.0.4"
 end