SLA for services

This change adds SLA functionality for services configured in Selena. This new functionality takes service history of each active service and counts SLA. Calculated SLA is stored in DB for every day. The data are displayed on a front page. There are 3 information available: SLA for 7 days, 1 month and 3 months. 
don't mix tzaware and naive datetimes unless you make sure they're both UTC - will break if executed at midnight. 
Also, would you try and calculate the 3 months SLA on 2014-05-31 this will end up with "ValueError: day is out of range for month"