Adds info about supported credit cards. Related to #74.

![europython_2014_-_purchase_tickets_form-2](https://f.cloud.github.com/assets/3782/2339134/ace182ae-a4b1-11e3-99e3-210e12dc2eda.png)
![europython_2014_-_purchase_tickets](https://f.cloud.github.com/assets/3782/2339135/afb614e0-a4b1-11e3-8450-b86cbbae2900.png)