Pebble fixes

Hi there!

We have noticed that Futura Weather can exhibit some behaviour that causes moderate or severe reductions in watch battery life. This PR aims to fix those.

In particular, this PR:
- Does not reload the weather image from flash every second
- Only updates the displayed weather information when it actually changes, instead of every clock tick
- Does not get stuck in a retry loop that consumes kills the battery in under a day when disconnected
- Reduces the tick interval from once per second to once per minute
- Makes use of the bluetooth service for retries and icon display
- Simplifies some logic

Please take a look and merge.

Thanks! :D