Airtime, Duty-Cycle and Fair-Use calculation package

[htdvisser]

No description provided.

[KtorZ]

There's already one -> https://github.com/TheThingsNetwork/ttn/tree/develop/core/dutycycle

What do you have in mind exactly ? ( we could rename it to have a meaning closer to what it actually does ... )

[htdvisser]

At the moment it's only used for gateway duty cycles. We should also use it on a node-level (for the fair use policy) and in ttnctl (to show how much airtime you're wasting). Therefore we might want to move the calculation part to a separate package.

[htdvisser]

This has been implemented in the refactor branch. See #199 for more info.