Currently set to the timestamp the website row was added to Mysql, it is often required to change the time in the past
we should add a parameter to Website create/update API functions to allow setting this date in the past
(In [4200]) Fixes #2191 the addSite / updateSite APIs now have a startDate parameter so it is possible to load logs that are in the past, and let Piwik know about it (without hacking the ts_created field in the DB)