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/httpful/examples/github.php

@@ -0,0 +1,9 @@
+<?php
+// XML Example from GitHub
+require(__DIR__ . '/../bootstrap.php');
+use \Httpful\Request;
+
+$uri = 'https://github.com/api/v2/xml/user/show/nategood';
+$request = Request::get($uri)->send();
+
+echo "{$request->body->name} joined GitHub on " . date('M jS', strtotime($request->body->{'created-at'})) ."\n";