Rework of wake-up interval cache

In this release there has been a complete rework of the wake-up interval cache, as well as some other small fixes.
There was a problem in the code that when a device was added, the cache would update the wake-up interval to an inpossible state. The device would set the value to the maximal interval, but the cache still wants to update the value, therefore getting stuck in a loop.
This loop is no longer possible.

I feel like this rework is not worth a patch version, therefore I arrived at version 0.2.0. Yay.
There is loads to do still and this is just a hobby project, so things are moving quite slowly.
Any patches, issues or pull requests are welcome :-)