Add recursion / factorial koan.

src/koans/recursion.clj

@@ -0,0 +1,21 @@
+(defn factorial [n]
+  __)
+
+(meditations
+  "Simple things may appear simple."
+  (= 1 (factorial 1))
+
+  "They may require other simple steps."
+  (= 2 (factorial 2))
+
+  "Sometimes a slightly bigger step is necessary"
+  (= 6 (factorial 3))
+
+  "And eventually you must think harder"
+  (= 24 (factorial 4))
+
+  "You can even deal with very large numbers"
+  (= (factorial 1000N) (factorial 1000N))
+
+  "But what happens when the machine limits you?"
+  (= (factorial 10000N) (factorial 10000N)))