For discussion -- Stripe integration

Summary:
various stripe stuff, including

- external stripe library
- payment form
- test controller to play with payment form, sample business logic

My main questions / discussion topics are...

- is the stripe PHP library too big? (ie should I write something more simple just for phabricator?)
-- if its cool, what is the best way to include the client? (ie should I make it a submodule rather than the flat copy here?)
- is the JS I wrote (too) ridiculous?
-- particularly unhappy with the error message stuff being in JS *but* it seemed the best choice given the most juicy error messages come from the stripe JS such that the overall code complexity is lowest this way.
- how should the stripe JS be included?
-- flat copy like I did here?
-- some sort of external?
-- can we just load it off stripe servers at request time? (I like that from the "if stripe is down, stripe is down" perspective)
- wasn't sure if the date control was too silly and should just be baked into the form?
-- for some reason I feel like its good to be prepared to walk away from Stripe / switch providers here, though I think this is on the wrong side of pragmatic

Test Plan: - played around with sample client form

Reviewers: epriestley

Reviewed By: epriestley

CC: aran

Differential Revision: https://secure.phabricator.com/D2096

externals/stripe-php/lib/Stripe/Util/Set.php

@@ -0,0 +1,34 @@
+<?php
+
+class Stripe_Util_Set
+{
+  private $_elts;
+
+  public function __construct($members=array())
+  {
+    $this->_elts = array();
+    foreach ($members as $item)
+      $this->_elts[$item] = true;
+  }
+
+  public function includes($elt)
+  {
+    return isset($this->_elts[$elt]);
+  }
+
+  public function add($elt)
+  {
+    $this->_elts[$elt] = true;
+  }
+
+  public function discard($elt)
+  {
+    unset($this->_elts[$elt]);
+  }
+
+  // TODO: make Set support foreach
+  public function toArray()
+  {
+    return array_keys($this->_elts);
+  }
+}