Touch file when deployment complete to help scripted workflows

[leonstafford]

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.