Add article repartition in stats

Add article repartition per hour, per day of week, per month for all feeds but also for individual feeds.
Add repartition statistic support in Sqlite
Small fixes to follow @marienfressinaud recommandations
Small fixes to follow @marienfressinaud recommandations (take 2)
Remove unnecessary title property
Merge pull request #548 from aledeg/more-stats

Add article repartition in stats