Return Guestbook interface instead of SQLiteDatabase instance.

database/sqlite/database.go

@@ -100,7 +100,7 @@ type SQLiteDatabase struct {
 	db *sql.DB
 }
 
-func New(filePath string) (*SQLiteDatabase, error) {
+func New(filePath string) (guestbook.Guestbook, error) {
 	db, err := sql.Open("sqlite3", dsn(filePath))
 	if err != nil {
 		return nil, err