move DateTime functions from Daemon class to Utility

About `Daemon::date_period` and `Daemon::date_period_text` methods
1) not in camel case style
2) not related with `Daemon` class (i think it's bad place for this methods)
3) support only timestamp 
4) already exists http://www.php.net/manual/en/datetime.diff.php
5) all this `$fin[2]`, `$st[0]` a bit ugly :-)