Introduce Composer support

Commerce 8.x is depending on https://www.drupal.org/project/composer_manager, rather than using libraries. The only thing is that css/js gets tricky. You have to just guess where it exits. See http://webmozarts.com/2014/12/03/puli-powerful-resource-management-for-php.

If it becomes too much of an issue, we might need to copy in the appropriate css into the D8 module to style the configuration form.

Commerce does it this way:
https://github.com/commerceguys/commerce/blob/8.x-2.x/composer.json

Here is the blog post:
https://drupalcommerce.org/blog/31782/managing-d8-module-dependencies-ne...

Added first attempt at support into https://github.com/heddn/htmlpurifier

NOTE: As of the commit in #6, the process to pull in the upstream library has changed from using libraries to composer_manager. The steps to pull in the library using composer are listed in this blog: https://drupalcommerce.org/blog/31782/managing-d8-module-dependencies-ne...

The new flow becomes:

1. Install Composer Manager, and initialize it using the init.sh script (or drush composer-manager-init).
2. This registers the module's Composer command for Drupal core.
3. Download the desired modules (such as Commerce).
4. Inside your core/ directory run composer drupal-update.
5. This rebuilds core/composer.json and downloads the new dependencies.
6. Install the modules.