Add missing Datapoint fields
Add missing Datapoint fields

The following fields were missing from decoding:

- `temperature_low`
- `temperature_low_time`
- `temperature_high`
- `temperature_high_time`

These were added by Darksky in late August of 2017. Details can be
found in this blog post:

http://status.darksky.net/2017/08/29/daily-updates.html
zeyla/darksky.rs