Improve and/or document ability to create mock Incident objects

Howdy!

Thanks for sharing this! I'm trying to write some unit test coverage (which is running in CI, so I'm mocking out the calls to pagerduty) but am having a rough time figuring out how to generate a mocked out `Incident` object. For a list of Incidents, I was trying something along the lines of:

```
Incidents(Incident(html_url='pagerduty.com/theincident', incident_key='this was a page'))
```

Since it looks like when you request a list of incidents they are wrapped in an `Incidents` (aka a `Collection`). I'm curious if the community already has a pattern for doing this, but if not I'll come back around in a few weeks with a path- either a test fixture, example, or something depending on what everyone prefers.

Thanks!
Joe