Summarize data

Please, check my code!
I forgot to include some kind of readme.st. To test custom command:
python manage.py summarize [date YYYY-MM-DD] [--nodeletion]
If you pass no parameters it summarizes current date minus 15 days (single day and no a 15-days period)
@lorenzogil :
> Also note that you are only computing averages but we also want means.

Lorenzo, isn't the mean the same as the average?
Maybe you wanted to say "median"?