Add binary search to Util

For some reason there doesn't appear to be a binary search function in
Elixir's standard library, so this implements that.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

test/contrib/markov/chain_test.exs

@@ -0,0 +1,12 @@
+defmodule MarkovChainTest do
+  use ExUnit.Case
+  alias Omnibot.Contrib.Markov.Chain
+
+  test "chain train_one works correctly" do
+    chain = %Chain {order: 2}
+            |> Chain.train_one(["foo", "bar"], "baz")
+    #assert chain.chain == [
+      #{["foo", "bar"], {"baz", 1}}
+      #]
+  end
+end