mk-wp-demo-site - Reorg

 * Move basic Civi/WP install to civilib.sh
 * Infer web-root based on $0
 * Move more constants into variables
mk-wp-demo-site/civilib - Extract variable CIVI_UF
civilib.sh - Import changes from civicrm-drupal-project
mk-wp-demo-site - Cleanup
mk-wp-demo-site - Move configuration steps to app/config/install-wp-demo.sh
Rename bin/mk-wp-demo-site to bin/civi-install
civi-install - Ensure that bundled commands (wp-cli, etal) are in PATH