Fix path for asset storage (#144)

Define locations for keys and secondary zones, 'n stuff.

Add a bunch of tests as well.

core/dns/storage.go

@@ -0,0 +1,28 @@
+package dns
+
+import (
+	"path/filepath"
+
+	"github.com/miekg/coredns/core/assets"
+)
+
+var storage = Storage(assets.Path())
+
+// Storage is a root directory and facilitates
+// forming file paths derived from it.
+type Storage string
+
+// Zones gets the directory that stores zones data.
+func (s Storage) Zones() string {
+	return filepath.Join(string(s), "zones")
+}
+
+// Zone returns the path to the folder containing assets for domain.
+func (s Storage) Zone(domain string) string {
+	return filepath.Join(s.Zones(), domain)
+}
+
+// SecondaryZoneFile returns the path to domain's secondary zone file (when fetched).
+func (s Storage) SecondaryZoneFile(domain string) string {
+	return filepath.Join(s.Zone(domain), "db."+domain)
+}