Add notification API

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.