add protocol register function

lib/mbtiles.js

@@ -65,6 +65,10 @@ function MBTiles(uri, callback) {
     return undefined;
 };
 
+MBTiles.registerProtocols = function(tilelive) {
+    tilelive.protocols['mbtiles:'] = MBTiles;
+};
+
 // Finds all mbtiles file in the filepath and returns their tilesource URI.
 MBTiles.list = function(filepath, callback) {
     filepath = path.resolve(filepath);