added binary search example -- chapter 6 of beautiful code

examples/beautiful_code/binary_search.coffee

@@ -0,0 +1,16 @@
+# Beautiful Code, Chapter 6.
+# The implementation of binary search that is tested.
+
+# Return the index of an element in a sorted list. (or -1, if not present)
+index: list, target =>
+  [low, high]: [0, list.length]
+  while low < high
+    mid: (low + high) >> 1
+    val: list[mid]
+    return mid if val is target
+    if val < target then low: mid + 1 else high: mid
+  return -1
+
+print(2 is index([10, 20, 30, 40, 50], 30))
+print(4 is index([-97, 35, 67, 88, 1200], 1200))
+print(0 is index([0, 45, 70], 0))