Touch file when deployment complete to help scripted workflows

Given a user with a custom deploy script outside of WordPress
When WP2Static calls `wp2static_post_deploy_trigger`
Then the file `wp2static-last-deploy` should be created
and the file should contain a timestamp

This will make it easier for watch scripts to trigger custom workflows and allow WP2Static to take care of detecting site changes, running detect, crawl and post_process jobs.