prevent maxlisteners error

we only attach with .once() and always .emit(). Debugging as shown that
there are no "memory leaks" with listeners being added infinitely.

lib/mbtiles.js

@@ -53,6 +53,7 @@ function MBTiles(uri, callback) {
 
     var key = url.format(uri);
     if (!cache[key]) {
+        this.setMaxListeners(0);
         cache[key] = this;
         this._open(uri);
     }