HUGE performance improvement for grouped queries

Added data aggregation:
1. create aggregate table using misc/sql/aggregation.sql
2. run initial aggregation using misc/sql/aggregation.sql
3. set $config['aggregate'] = true in etc/volkszaehler.conf.php
4. setup CRON to run delta aggregation using misc/sql/aggregation.sql
Merge branch 'aggregate' of https://github.com/andig/volkszaehler.org into aggregate