added binary search example -- chapter 6 of beautiful code

examples/beautiful_code/quicksort_runtime.coffee

@@ -0,0 +1,13 @@
+# Beautiful Code, Chapter 3.
+# Produces the expected runtime of Quicksort, for every integer from 1 to N.
+
+runtime: N =>
+  [sum, t]: [0, 0]
+  for n in [1..N]
+    sum += 2 * t
+    t: n - 1 + sum / n
+  t
+
+print(runtime(3) is 2.6666666666666665)
+print(runtime(5) is 7.4)
+print(runtime(8) is 16.92142857142857)