Add support for admin_menu

There's a case in which users may want to have tweets of their accounts displayed immediately on the site. For people using admin_menu, I think it would be a nice feature to have a link to flush the twitter pull cache, next to the other items within the "Flush all caches" item of admin_menu.

Patch attached. It's just the implementation of hook_admin_menu_cache_info() and a callback function to flush the caches. It also fixes several empty lines at the end of twitter_pull.module, leaving only one newline at the end.

Cheers,

Salva