Cron would not clear cache

Thank You guys for this module, it's my last resort on client's windows machine.

Issue ies: cron does not clean cache because global variable $_boost is not populated at hook_init (function's logic does not apply for cron.php).
instead I directly replaced it with function that populates that variable.

Here's patch I created against 7.x-1.x branch, I hope it works, it's my first patch ;)