Update dependencies / Fix package.json and .jshintrc

* I updated [accord](https://github.com/jenius/accord), [gulp-util](https://github.com/gulpjs/gulp-util) and [through2](https://github.com/rvagg/through2).
* I updated `.jshintrc`.
  * I enabled `unused` option.
  * I removed all globals. They are already defined by [`mocha` option](https://github.com/jshint/jshint/blob/bc6e97ebc3baff09eda34a3f4bd0ed86ccf84dbd/src/vars.js#L649-L662).
* <del>I removed `.npmignore` and add `files` field to package.json instead.</del>