Need Events API v2 Support

Need to support Events API v2 (not to be confused with REST API v2). New Events API comes with new Common Event Format payload requirements (PD-CEF). Need to create a `Events()` implementation to handle the new requirements.

README should also be updated for reference to the [PD-CEF](https://support.pagerduty.com/docs/pd-cef) & [Events API v2](https://v2.developer.pagerduty.com/docs/events-api-v2) documentation.