Support using MySQL without a password

Fixes #568

app.go

@@ -569,8 +569,8 @@ func (app *App) InitDecoder() {
 // tests the connection.
 func ConnectToDatabase(app *App) error {
 	// Check database configuration
-	if app.cfg.Database.Type == driverMySQL && (app.cfg.Database.User == "" || app.cfg.Database.Password == "") {
-		return fmt.Errorf("Database user or password not set.")
+	if app.cfg.Database.Type == driverMySQL && app.cfg.Database.User == "" {
+		return fmt.Errorf("Database user not set.")
 	}
 	if app.cfg.Database.Host == "" {
 		app.cfg.Database.Host = "localhost"