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)
I think the `i+1` looks better without spaces, because the `==` has lower priority.

See rationale in [this section of PEP 8](http://legacy.python.org/dev/peps/pep-0008/#other-recommendations).