Add Composer support

[Composer](http://getcomposer.org/) is a package management system for PHP. This PR allows Normalize.css to be installed seamlessly into projects that use Composer. This is quite similar to how you can install Normalize.css using [Bower](http://twitter.github.io/bower/).

The following is an example of a *composer.json* file that would download and install Normalize.css:

``` json
{
    "require": {
        "necolas/normalize.css": "*"
    },
    "minimum-stability": "dev"
}
```

The above is processed by running a composer install:
``` bash
$ curl -sS https://getcomposer.org/installer | php
$ composer.phar install
```