Check and fix code style and formatting

I added gulp to run tasks on this project. The `lint` tasks includes [JSHint](http://www.jshint.com/docs/) to check for common error in JavaScript code and [`jscs`](https://github.com/mdevils/node-jscs) to check for proper formatting. I think it's really useful to have clear guidelines for the code, it makes it easier to read and to contribute in general.