Add Stripe_RateLimitError that extends Stripe_InvalidRequestError

To allow users to catch rate limit errors specifically.

r? @michelle 

Tested manually:
```text

Fatal error: Uncaught exception 'Stripe_RateLimitError' with message 'Request rate limit exceeded' in /Users/jim/stripe/stripe-php/lib/Stripe/ApiRequestor.php:137
Stack trace:
#0 /Users/jim/stripe/stripe-php/lib/Stripe/ApiRequestor.php(202): Stripe_ApiRequestor->handleApiError('{?  "error": {?...', 400, Array)
#1 /Users/jim/stripe/stripe-php/lib/Stripe/ApiRequestor.php(104): Stripe_ApiRequestor->_interpretResponse('{?  "error": {?...', 400)
#2 /Users/jim/stripe/stripe-php/lib/Stripe/ApiResource.php(102): Stripe_ApiRequestor->request('get', '/v1/charges', Array)
#3 /Users/jim/stripe/stripe-php/lib/Stripe/Charge.php(26): Stripe_ApiResource::_scopedAll('Stripe_Charge', Array, NULL)
#4 /Users/jim/php_test/chunder.php(8): Stripe_Charge::all(Array)
#5 {main}
  thrown in /Users/jim/stripe/stripe-php/lib/Stripe/ApiRequestor.php on line 137
```