Return empty hash from list if directory doesn't exist.

Conflicts:

	test/list.test.js

lib/mbtiles.js

@@ -76,6 +76,7 @@ MBTiles.registerProtocols = function(tilelive) {
 MBTiles.list = function(filepath, callback) {
     filepath = path.resolve(filepath);
     fs.readdir(filepath, function(err, files) {
+        if (err && err.code === 'ENOENT') return callback(null, {});
         if (err) return callback(err);
         for (var result = {}, i = 0; i < files.length; i++) {
             var name = files[i].match(/^([\w-]+)\.mbtiles$/);