Add WP-CLI compatibility

Without this PR, the plugin is not able to be activated with WP-CLI as there is a fatal error regarding a class not being defined.

 * Log actions performed by WP-CLI anonymously (without `--user`) as entries with user=0
 * Note in Stream meta any actions performed by WP-CLI with a `--user` supplied.
 * Show admin notices via the WP-CLI logger instead of via the `all_admin_notices` hook
 * Add WP-CLI in the author drop-down
 * Attribute actions by WP-CLI with `--user` as “via WP-CLI” in the table.
 * Fix uninstall routine and DB version