Add supported fields to POST /incident request.

[archydragon]

Add fields listed at https://api-reference.pagerduty.com/#!/Incidents/post_incidents

[electrofelix]

@stmcallister I see there are two PRs for enabling the ability to create incidents with Urgency, this one and #147, would it be possible to have one of them land (once the conflict is resolved)?

[stmcallister]

@electrofelix yep. i'll look at these right now and see if we can get one of them merged.

[stmcallister]

Thank you for making this contribution! This looks good to me.

[stmcallister]

This mostly looks. I made a minor change where fields of type APIReference and APIDetails need to remain pointers. Thanks for the contribution!