[3h]. Implement Statistic module.

[yev-kanivets]

1. Calculate statistic for every day.
2. Send an email with gathered statistic once per day.

Statistic is the table with following information for every RSS link:

• number of update requests.
• number of failed update requests.
• number of new articles that were fetched and saved.
• number of new articles that were saved.

Then just add one more line with sums for every column.