Add path.join so that pkg could auto detect assets (#80)

* Add path.join so that pkg could auto detect assets

* Delete require utils

lib/mbtiles.js

@@ -24,7 +24,7 @@ function hash(z, x, y) {
 // MBTiles class for doing common operations (schema setup, tile reading,
 // insertion, etc.)
 module.exports = MBTiles;
-MBTiles.schema = fs.readFileSync(__dirname + '/schema.sql', 'utf8');
+MBTiles.schema = fs.readFileSync(path.join(__dirname + '/schema.sql'), 'utf8');
 
 // Provides access to an mbtiles database file.
 // - uri: A parsed URL hash, the only relevant part is `pathname`.