Add API for operation with modal (simple) dialogs

Add an official notification API for handling alerts, confirms and prompts. See README for API usage.

The default naming is singular because Capybara ships with a Selenium driver which can only deal with a single alert/confirm/prompt at a time. While other drivers can support multiple notifications, I figured it would be best to stay consistent with Capybara core. I did, however, alias the methods in their plural form so that they would read better when dealing with multiple notifications.