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/example/test-composer.php

@@ -0,0 +1,14 @@
+<?php
+
+// run this file to test your composer install of Balanced
+
+require(__DIR__ . '/vendor/autoload.php');
+
+\Httpful\Bootstrap::init();
+\RESTful\Bootstrap::init();
+\Balanced\Bootstrap::init();
+
+echo "[ OK ]\n";
+echo "balanced version -- " . \Balanced\Settings::VERSION . " \n";
+echo "restful version -- " . \RESTful\Settings::VERSION . " \n";
+echo "httpful version -- " . \Httpful\Httpful::VERSION . " \n";