Add Composer support

[Composer](http://getcomposer.org/) is a package management system for PHP. This pull request allows PHP projects to depend on, download and install jQuery Tags Input. Once available, you could stick `xoxco/jquery-tags-input` in your own *composer.json* file, run `composer.phar install` and have jQuery Tags Input be downloaded for you.

Thank you, and great work with this jQuery plugin. It's great!