Clear achievements after displaying notifications

Rather than clear these when viewing the achievement page, clear them after the notification has been shown.

Also, use the standard clear notification function.

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Apologies for the newline at the end of the file, GitHub's editor appears to have added that. If you'd like, I can repush this.

Alternative solution: implement an Ajax handler for when the background is clicked, or add an action so a plugin can do this.

This fixes the following .org support requests:

* [Achievement notification always shows up](http://wordpress.org/support/topic/achievement-notification-always-shows-up)
* [Achievement box won't go away...](http://wordpress.org/support/topic/achievement-box-wont-go-away?replies=2)
* [Popup Bug Report](http://wordpress.org/support/topic/bug-report-13)