CheckItem missing greater than free features

[doug4j]

For a a trello account that greater than free, there is the option to set/get the member, due date, and DueReminder for a Trello CheckList CheckItem. I have implemented this in a private repo. I found adding this very useful for my needs. I'd like to explore bringing this work into the adlio / trello project. It was pretty straight-forward.

type CheckItem struct {
       //... other fields
        IDMember    string     `json:"idMember,omitempty"` // >free feature only
	Due         time.Time  `json:"due,omitempty"` // >free feature only
	DueReminder int        `json:"dueReminder,omitempty"` //>free feature only
}