Time-scheduled target temperature changes

[amorphic]

It would be nice if a user could schedule a target temperature change for some time in the future. I.e. 'At 10:00 on YYYY-MM-DD set target temperature to 10C'.

This might be useful for use cases such as lagering beer, where target temperatures change throughout the brewing cycle.

Will require #10 to be completed. Will also need the ability for scheduled changes to be stored and processed in the braubuddy engine as well as being added/updated by the API.