Add calendar caching

[tykeal]

There have been several times where for whatever reason Airbnb returned back an empty calendar (or more likely a 500 error) and the event calendar got wiped and then came back a few minutes later. This can potentially cause the event sensors to clear programmed door codes and then put them back. A knock on effect is that if Keymaster has issues with programming the lock the working code could suddenly stop working, or even worse, an overridden code is now reset to something different.