Change color schema on critical battery level

[guckuck]

Change the behavior on pebble 2 with black white display: Now the
colors will be inverted when battery level is <= 20%.

[guckuck]

Thanks for this nice watchface. On my new pebble 2 with black white display the color orange is suboptimal. Now I use inverted colors. Maybe this is interesting for others too.

Disclaimer: This is my first pull request and my first pebble modification.

[cfg1]

The code looks OK, but I would like to test it first on all other platforms, which takes time. And I have almost no time atm.

[guckuck]

Thank you. I tested it with the emulator, not with real watches.