Add MergeIncidents (using Incident)

[atomicules]

Allows for merging of incidents as follows:

incidents := make([]string, 3)
incidents[0] = "PXXXXXX"
incidents[1] = "PYYYYYY"
incidents[2] = "PZZZZZZ"

first := incidents[0]
rest := incidents[1:]

tomerge := make([]pagerduty.APIObject, 0)

for i := range rest {
    inc := pagerduty.Incident{
	APIObject: pagerduty.APIObject{
	    ID:   rest[i],
	    Type: "incident_reference",
	},
    }
    tomerge = append(tomerge, inc)
}
result := client.MergeIncidents(email, first, tomerge)

Note that this option (using pagerduty.Incident most closely matches the existing ManageIncident, but since Merge literally only requires the id and the type of "incident_reference" I've also got an option 2 branch which uses pagerduty.APIObject; I personally think that option 2 is a bit cleaner for actual creation of "objects" for merging, but I guess doesn't sit as well with the existing ManageIncidents function.