Adding 8.x variant

[derhasi]

In an 8.x branch we can provide an example for Drupal 8. Maybe based on https://github.com/tstoeckler/drupal-site-template . Maybe we do not need the subtree split and use drupal/drupal:8.* from our drupal packagist.

[webflo]

I tried to update the site template against latest Drupal 8 beta and Drush. The current version is available on https://github.com/webflo/drupal-site-template/tree/dev

[webflo]

Merged the latest code from webflo/drupal-site-template. Huge thanks to @tstoeckler for the initial version.

@tstoeckler Please review the 8.x branch if i missed anything.