Refs #2327

 * Adding new archive.php optimized rewrite of archive.sh - see description below
 * Adding new API to return only active website ID with visits since $timestamp (which is used to get all sites with visits since last archive execution)
{{{
 * Description
 * This script is a much optimized rewrite of archive.sh in PHP 
 * allowing for more flexibility and better performance when Piwik tracks thousands of websites.
 * 
 * What this script does:
 * - Fetches Super User token_auth from config file
 * - Calls API to get the list of all websites Ids with new visits since the last archive.php succesful run
 * - Calls API to get the list of segments to pre-process
 * The script then loops over these websites & segments and calls the API to pre-process these reports.
 * It does try to achieve Near real time for "daily" reports, processing them as often as possible.
 * 
 * Notes about the algorithm:
 * - To improve performance, API is called with date=last1 whenever possible, instead of last52 
 * - The script will only process (or re-process) reports for Current week / Current month  
 * 	 or Current year at most once per hour. 
 *   You can change this timeout as a parameter of the archive.php script.
 *   The concept is to archive daily report as often as possible, to stay near real time on "daily" reports,
 *   while allowing less real time weekly/monthly/yearly reporting. 
 */

/**
 * TODO/Ideas
 * - Process first all period=day, then all other periods (less important)
 * - Ensure script can only run once at a time
 * - Add "report last processed X s ago" in UI grey box "About"
 * - piwik.org update FAQ / online doc
 * - check that when ran as crontab, it will email errors when there is any
}}}

git-svn-id: http://dev.piwik.org/svn/trunk@5087 59fd770c-687e-43c8-a1e3-f5a4ff64c105