Add download counter on each book

* Closes: #40

lib/database/stats.go

@@ -40,6 +40,15 @@ func (db *pgDB) IncDownloads(ID string) error {
 	return err
 }
 
+func (db *pgDB) GetDownloadCounter(ID string) (int, error) {
+	var num int
+	err := db.sql.Model(&Visit{}).
+		Column("downloads").
+		Where("book_id = ?", ID).
+		Select(&num)
+	return num, err
+}
+
 func (db *pgDB) GetFrontPage() FrontPage {
 	return db.frontPage
 }