Add logic to check if it is dark out for the user

* Found suncalc library, which calculates sunset and sunrise
  * Based on user's lat, long, and the cuurent date
* Now, if the ISS is overhead, we also check to see if it is
  dark out
  * If it is, then a different message is displayed
Merge branch 'master' into sunset-messages

Conflicts:
	src/js/pebble-js-app.js
Merge pull request #14 from hwhelchel/sunset-messages

Sunset messages