Determine sunrise/sunset

[leighghunt]

Use location to determine sunrise/sunset time, and allow configuration to declare offset for setting time camera will turn on/off.

E.g. - `config.json'

    "daytime_starts_at_h": 6,
    "daytime_ends_at_h": 18,

becomes something like:

    "daytime_starts_at_h": sunrise-30m,
    "daytime_ends_at_h": sunset+1h,

Here's a good thread on some libraries: https://stackoverflow.com/questions/38986527/sunrise-and-sunset-time-in-python

Some other thoughts:

• We could possibly use lat/lon, or perhaps a city name in config to work out times. City name might be easier to configure.