Support for V2 of the API

[stephenyeargin]

Related to #98

It sounds like new users of PagerDuty won't be able to generate V1 API keys after 4/24, and it will stop working in October. I did some of the most recent work on this package, but I now work for a group that isn't paying for PagerDuty.

Before this becomes a 😡 issue for users, does anyone have some bandwidth (and a PD account) to look at spiking something together? Maybe something already has in use?

Reference: https://v2.developer.pagerduty.com/docs/migrating-to-api-v2

[stephenyeargin]

Released v3.0.0 this morning that reportedly works with the new API versions. I have a feeling that there are some gaps in features (perhaps Webhooks), but this gets the core functionality back in place.