Update initialisms / acronyms to be capitalized

[theckman]

There are some names in the packages that are either initialisms or acronyms, and they are not capitalized. That is the common style, and is called out in the Go wiki, and so it seems good to enforce it here.

This will definitely be a breaking change, and so we should wait to do that until v2.0.0.

Example:

go-pagerduty/incident.go

Line 85 in eab6a10

Id string `json:"id,omitempty"`

[theckman]

We need to remove this Id field anyhow because of the issue reported in #218, and so we are going to move this into v1.5.0.