AppMessageHandler for Modern Watchface Weather code

I added Weather support for the Modern Watchface (http://forums.getpebble.com/discussion/4540/watch-face-modern-analog-update-v3-1/p1)

Currently "Enabled" is hardcoded but we could use the Launcher Endpoint 0x49 (which the watchface fires if its activated/deactivated)

Also the "C" for celsius is hardcoded as-well, not sure where to put the app settings.