Work towards #56: Add permalink to every pattern

This makes cleaner URLs thanks to jekyll

memento/index.md

@@ -2,6 +2,7 @@
 layout: pattern
 title: Memento
 folder: memento
+permalink: /patterns/memento/
 categories: pattern_cat
 tags: pattern_tag
 ---
@@ -18,4 +19,4 @@ object's internal state so that the object can be restored to this state later.
 
 **Real world examples:**
 
-* [java.util.Date](http://docs.oracle.com/javase/8/docs/api/java/util/Date.html)
+* [java.util.Date](http://docs.oracle.com/javase/8/docs/api/java/util/Date.html)