Adds arrayBuffer and buffer definitions. (#16)

This is based entirely on the examples in the readme.  If the examples in the readme are incorrect then this definition is also incorrect.

index.d.ts

@@ -42,6 +42,20 @@ interface Hash {
    */
   (message: Message): string;
 
+  /**
+   * Hash and return ArrayBuffer.
+   *
+   * @param message The message you want to hash.
+   */
+  arrayBuffer(message: Message): ArrayBuffer;
+
+  /**
+   * Hash and return ArrayBuffer.
+   *
+   * @param message The message you want to hash.
+   */
+  buffer(message: Message): ArrayBuffer;
+
   /**
    * Create a hash object.
    */