- ad.install

    o introduce adgroup into ad_statistics and ad_clicks
 - ad.module
    o track ad group on redirect
    o track ad group in statistics, when applicable
    o validate aid, group and hostid on redirect
 - serve.php
    o track ad group
    o support passing in url where ads are displayed
 - ad_cache_memcache.module
    o sync ads not in any group along with ads in groups
    o track ad group in statistics
 - ad_cache_memcache.inc
    o track ad group in statistics
    o add additional debug