Use 'xyz' as scheme in metadata

As getTile swaps the z-coordinate from tms to xyz

lib/mbtiles.js

@@ -234,7 +234,7 @@ MBTiles.prototype.getInfo = function(callback) {
 
     var mbtiles = this;
     var info = {};
-    info.scheme = 'tms';
+    info.scheme = 'xyz';
     info.basename = path.basename(mbtiles.filename);
     info.id = path.basename(mbtiles.filename, path.extname(mbtiles.filename));
     info.filesize = mbtiles._stat.size;