Add a date range filter

[prytoegrian]

Cf. #57

Now, for the route /attack, availables filters are :

• status,
• created_before,
• created_after.

Based on the example in db:createFilterChain, I simply added new filters. I also refactored testing cases, for improving readability.

My golang is far from perfect, please point me my mistakes if there are any.

I would like to avoid parsing AttackInfo.CreatedAt ; it would be easier if it were time.Time. For a future PR, i'll try to change its type.

Thanks !

[prytoegrian]

Ready to review.