Noramlize Store interface

Instead of exposing a key/value get/set style interface, design the
interface around the actions that are actually required.

This is a first step towards supporting adding a Store to support
Google Cloud's Datastore.

storage/store.go

@@ -1,10 +1,23 @@
 package storage
 
-// Store defines a standard interface for storage.
+import "errors"
+
+// ErrNotFound is the default error message when data is not found.
+var ErrNotFound = errors.New("not found")
+
+// Store defines a standard interface for storage
 type Store interface {
 	Close() error
-	Get([]byte) ([]byte, error)
-	Set([]byte, []byte) error
-	Delete([]byte) error
+	Create(UID []byte, URL []byte) (*Record, error)
+	FindByUID(UID []byte) (*Record, error)
+	FindByURL(URL []byte) (*Record, error)
+	DeleteByUID(UID []byte) (*Record, error)
+	DeleteByURL(URL []byte) (*Record, error)
 	NextSequence() (int, error)
 }
+
+// Record provides a standard way to refer to a shortened URL.
+type Record struct {
+	UID []byte
+	URL []byte
+}