Add missing annotation on Contacts property in ContactList

[lkabell]

Why?

The Contacts list would always be empty because of the missing JSON annotation on the Contacts field, e.g. when doing

contactList, err := ic.Contacts.List(intercom.PageParams{Page: 2})
contactList.Contacts

contactList.Contacts would be empty.

How?

Added JSON annotation.

[taipeicoder]

Same goes for other resources (e.g: CompanyList)