Make temperature_scale getters and setters use symbols

Fixes https://github.com/ericboehs/nest_thermostat/issues/15. However, `temperature_scale=` now raises an exception if an invalid argument is passed in, so this PR would mean a major version update. If you don't want to do that, I'll gladly make it support the old behavior :)
You can omit the code markings and just use plain english words (to match "fahrenheit") rather than symbols. The code examples below demonstrate how to change the unit adequately.