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)
This is wrong, list comprehensions shouldn't have side effects. I strongly recommend to turn this into a loop:

```python
for col in connection.get_db().collection_names():
    if col.endswith(self.RAW_SUFFIX):
        collections.append(col)
```

It's more readable too IMO.