Tracking WordPress Manual/Auto Update

Resolves #80, @fjarrett please review.

For the reference, specially for the upcoming unit tests, to initiate the auto update process, the following filters/actions is needed:
```php
add_action('admin_init', 'wp_maybe_auto_update');
add_filter( 'allow_major_auto_core_updates', '__return_true' );
add_filter( 'automatic_updates_is_vcs_checkout', '__return_false' );
delete_option( 'auto_updater.lock' );
```
Plus changing the WP version itself via changing the global `wp_version` or editing the `version.php` file directly.