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/restful/src/RESTful/Page.php

@@ -0,0 +1,72 @@
+<?php
+
+namespace RESTful;
+
+class Page
+{
+    public $resource,
+        $total,
+        $items,
+        $offset,
+        $limit;
+
+    private $_first_uri,
+        $_previous_uri,
+        $_next_uri,
+        $_last_uri;
+
+    public function __construct($resource, $uri, $data = null)
+    {
+        $this->resource = $resource;
+        if ($data == null) {
+            $client = $resource::getClient();
+            $data = $client->get($uri)->body;
+        }
+        $this->total = $data->total;
+        $this->items = array_map(
+            function ($x) use ($resource) {
+                return new $resource($x);
+            },
+            $data->items);
+        $this->offset = $data->offset;
+        $this->limit = $data->limit;
+        $this->_first_uri = property_exists($data, 'first_uri') ? $data->first_uri : null;
+        $this->_previous_uri = property_exists($data, 'previous_uri') ? $data->previous_uri : null;
+        $this->_next_uri = property_exists($data, 'next_uri') ? $data->next_uri : null;
+        $this->_last_uri = property_exists($data, 'last_uri') ? $data->last_uri : null;
+    }
+
+    public function first()
+    {
+        return new Page($this->resource, $this->_first_uri);
+    }
+
+    public function next()
+    {
+        if (!$this->hasNext()) {
+            return null;
+        }
+
+        return new Page($this->resource, $this->_next_uri);
+    }
+
+    public function hasNext()
+    {
+        return $this->_next_uri != null;
+    }
+
+    public function previous()
+    {
+        return new Page($this->resource, $this->_previous_uri);
+    }
+
+    public function hasPrevious()
+    {
+        return $this->_previous_uri != null;
+    }
+
+    public function last()
+    {
+        return new Page($this->resource, $this->_last_uri);
+    }
+}