Fix _estimate to only count 3 days.
Fix _estimate to only count 3 days.

Scifabric/pybossa,stefanhahmann/pybossa,Scifabric/pybossa,geotagx/pybossa,jean/pybossa,harihpr/tweetclickers,stefanhahmann/pybossa,jean/pybossa,PyBossa/pybossa,inteligencia-coletiva-lsd/pybossa,inteligencia-coletiva-lsd/pybossa,OpenNewsLabs/pybossa,geotagx/pybossa,OpenNewsLabs/pybossa,harihpr/tweetclickers,PyBossa/pybossa