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)
For some things (host_mean_response_time) you are not computing that on the loop but computing it using a separate function that will iterate over the host_response_time_list at the end of the loop.

Now, for other things like host_max_response_time you are doing it inside the loop instead of using the max/min functions at the end of the loop.

I would prefer the pattern of using aggregate functions at the end of the loop. It's less performant but more readable. It just increase the constant k in the O(n) notation so performance wise, it's not a big deal in my opinion.

This way you won't need the ```host_max_response_time``` and ```host_min_response_time``` variables.