Add recursive binary search for ruby (ch. 1) and improve recursive max for ruby (ch. 4) (#113)

* add recursive binary search for ruby

* account for cases with empty lists and lists with only one item

* use p instead of print to show nil value

* add newline at end of file

01_introduction_to_algorithms/ruby/02_recursive_binary_search.rb

@@ -0,0 +1,26 @@
+def binary_search(list, item, low = 0, high = list.length - 1)
+  while low <= high
+    mid = (low + high) / 2
+    guess = list[mid]
+
+    if guess == item
+      return mid
+    elsif guess > item
+      high = mid - 1
+      binary_search(list, item, low, high)
+    else
+      low = mid + 1
+      binary_search(list, item, low, high)
+    end
+  end
+
+  # If item is not found
+  return nil
+end
+
+# Create array with numbers 1 through 100
+my_list = (1..100).to_a
+
+puts binary_search(my_list, 2) # => 1
+puts binary_search(my_list, 50) # => 49
+p binary_search(my_list, 101) # => nil