[wip] Instantiated services

[ob-stripe]

cc @remi-stripe

Pretty experimental yet, but it works. I will polish this up and split it into smaller PRs.

Sample syntax:

require_once('vendor/autoload.php');

// Instantiate client
$stripe = new \Stripe\StripeClient("sk_test_123");

// Retrieve method
$coupon = $stripe->coupons->retrieve("mycoupon");

// Create method, namespaced resource
$issuingCard = $stripe->issuing->cards->create(["authorization_controls" => ["max_amount" => 123]]);

// Custom method, namespaced resource
$details = $stripe->issuing->cards->details("ic_123");

// Update method, nested polymorphic resource
$updatedPaymentSource = $stripe->paymentSources->update("cus_123", " card_123", ["metadata" => ["key" => "value"]]);

[ob-stripe]

Closing in favor of #771.