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)
It's important to keep your imports in order. This is the guidelines we follow:

- One import per line. Instead of ```from datetime import date, datetime, timedelta``` we split that into 3 separate lines
- Group the imports in three different blocks and in each blocks sort them in alphabetically order:
   - First standard python library imports (collections, datetime, logging, optparse, sys, ..)
   - Then, third party libraries (django, mongoengine, numpy)
   - Finally, your project imports (okapiweb)