Add `where` method to models and lambda conditions

[gi0baro]

Something like:

Event.where(lambda e: (e.start_at >= datetime(2015, 5, 20)) & (e.ends_at < datetime(2015, 6, 20)))

instead of

db((Event.start_at >= datetime(2015, 5, 20)) & (Event.ends_at < datetime(2015, 6, 20)))