Remove Gruntfile / Remove unused variables

* I removed unused Gruntfile.js.
* I updated [opn](https://www.npmjs.org/package/opn), [http-proxy](https://www.npmjs.org/package/http-proxy) and [socket.io-client](https://www.npmjs.org/package/socket.io-client).
* I added [`mocha` option](https://github.com/jshint/jshint/blob/master/src/vars.js#L649-L662) to .jshintrc. It defines the global variables of [Mocha](http://visionmedia.github.io/mocha/), such as `describe` and `it`.
* I updated jshint task target.
* I removed all unused variables.
* I updated [the contributor list](https://github.com/shakyShane/browser-sync#contributors) using `gulp contribs` command.