add close method

lib/mbtiles.js

@@ -250,6 +250,15 @@ MBTiles.prototype._metadata = function(key, callback) {
         });
 };
 
+
+MBTiles.prototype._close = function(callback) {
+    this._db.close(callback);
+};
+
+MBTiles.prototype.close = function(callback) {
+    this._close(callback);
+};
+
 MBTiles.prototype._getInfo = function(callback) {
     var mbtiles = this;
     var info = {};