Add "Memento" recipe to the "Design Patterns" chapter.

Author: thirdtruck

chapters/design_patterns/memento.textile

@@ -0,0 +1,45 @@
+---
+layout: recipe
+title: Memento Pattern
+chapter: Design Patterns
+---
+
+h2. Problem
+
+You want to anticipate the reversion of changes to an object.
+
+h2. Solution
+
+Use the "Memento pattern":http://en.wikipedia.org/wiki/Memento_pattern to track changes to an object.  The class using the pattern will export a _memento_ object stored elsewhere.
+
+If you have application where the user can edit a text file, for example, they may want to undo their last action.  You can save the current state of the file before the user changes it and then roll back to that at a later point.  
+
+{% highlight coffeescript %}
+class PreserveableText
+	class Memento
+		constructor: (@text) ->
+
+	constructor: (@text) ->
+	save: (newText) ->
+		memento = new Memento @text
+		@text = newText
+		memento
+	restore: (memento) ->
+		@text = memento.text
+
+pt = new PreserveableText "The original string"
+pt.text # => "The original string"
+
+memento = pt.save "A new string"
+pt.text # => "A new string"
+
+pt.save "Yet another string"
+pt.text # => "Yet another string"
+
+pt.restore memento
+pt.text # => "The original string"
+{% endhighlight %}
+
+h2. Discussion
+
+The Memento object returned by _PreserveableText#save_ stores the important state information separately for safe-keeping.  You could even serialize this Memento in order to maintain an "undo" buffer on the hard disk or remotely for such data-intensive objects as edited images.

wanted-recipes.textile

@@ -145,7 +145,6 @@ h2. Design patterns
 ** Interpreter
 ** Iterator
 ** Mediator
-** Memento
 ** Observer
 ** State
 ** Template Method