Implement a DNS zone

Full implementation, DNS (and in the future DNSSEC). Returns answer in a
hopefully standards compliant way.
Testing with my miek.nl zone are included as well.
This should correctly handle nodata, nxdomain and cnames.

middleware/file/zone.go

@@ -0,0 +1,26 @@
+package file
+
+import (
+	"github.com/miekg/coredns/middleware/file/tree"
+
+	"github.com/miekg/dns"
+)
+
+type Zone struct {
+	SOA  *dns.SOA
+	SIG  []*dns.RRSIG
+	name string
+	*tree.Tree
+}
+
+func NewZone(name string) *Zone {
+	return &Zone{name: dns.Fqdn(name), Tree: &tree.Tree{}}
+}
+
+func (z *Zone) Insert(r dns.RR) {
+	z.Tree.Insert(r)
+}
+
+func (z *Zone) Delete(r dns.RR) {
+	z.Tree.Delete(r)
+}