Add support for Composer & publish to Packagist

See #149

I have only recently started using Composer and before today I have not prepared a component for publishing on Packagist. However, the following `composer.json` does clone the alertify.js repository into `components\alertify.js` directory:
```json
{
  "repositories": [
    {
      "type": "vcs",
      "url": "https://github.com/andrew-kzoo/alertify.js"
    }
  ],
  "require": {
    "components/alertify.js": "dev-master"
  }
}
```
Publishing to Packagist is straightforward. Few, if any, additional changes to `composer.json` should be necessary. http://getcomposer.org/doc/02-libraries.md#publishing-to-packagist