Add bitsDiff function.

src/algorithms/math/bits/bitsDiff.js

@@ -0,0 +1,13 @@
+import countSetBits from './countSetBits';
+
+/**
+ * Counts the number of bits that need to be change in order
+ * to convert numberA to numberB.
+ *
+ * @param {number} numberA
+ * @param {number} numberB
+ * @return {number}
+ */
+export default function bitsDiff(numberA, numberB) {
+  return countSetBits(numberA ^ numberB);
+}