Add Composer support

[RobLoach]

Composer 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.

The following is an example of a composer.json file that would download and install Normalize.css:

{
    "require": {
        "necolas/normalize.css": "*"
    },
    "minimum-stability": "dev"
}

The above is processed by running a composer install:

$ curl -sS https://getcomposer.org/installer | php
$ composer.phar install

[necolas]

Thanks but it looks like people can still install normalize.css without this project including an extra manifest file. I'm going to stick with a single manifest for a generic front-end package manager like Bower.

[RobLoach]

Sounds good with me! Thanks @necolas.