Stripe Errors

* Uses `drupal_set_message` to display errors returned from Stripe instead of `form_set_error`.  

* Adds more optional testing query string parameters to make for easier testing.

![screen shot 2014-11-18 at 9 29 50 am](https://cloud.githubusercontent.com/assets/1236811/5089036/410bf1b6-6f06-11e4-983f-e2fdfc5da232.png)
![screen shot 2014-11-18 at 9 30 19 am](https://cloud.githubusercontent.com/assets/1236811/5089039/4422299c-6f06-11e4-8d85-862f56b0acb9.png)
![screen shot 2014-11-18 at 9 29 18 am](https://cloud.githubusercontent.com/assets/1236811/5089030/39fe248e-6f06-11e4-9191-6d805ccf6f4a.png)