Add Operator support for date values

[dillan]

It would be very helpful to include Operator support for date values. For example, in the Kitura Query ORM, the QueryParams might look like:

struct ObjectParameters: QueryParams {
    var createdDate: InclusiveRange<Date>?
    var modifiedDate: InclusiveRange<Date>?
    var name: String?
    var type: String?
}