Add Balanced Payments API

Summary: Adds the Balanced PHP API to externals/. Ref T2787.

Test Plan: Used in next diff.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran, aurelijus

Maniphest Tasks: T2787

Differential Revision: https://secure.phabricator.com/D5764

externals/balanced-php/bootstrap.php

@@ -0,0 +1,4 @@
+<?php
+
+require(__DIR__ . '/src/Balanced/Bootstrap.php');
+\Balanced\Bootstrap::init();