create new zone method

[inamvar]

create new zone functionality added to zones servrice as Create method.

usage:

domainName :="example.com"
zone := cloudns.NewZone(domainName, cloudns.ZoneTypeMaster,
		[]string{
			"a.ns.customns.com",  //optional
			"b.ns.customns.com", // optional
                         },
		"")

res, err := client.Zones.Create(context.TODO(), zone)