Save and load gauge configuration

First pass at saving gauges. I don't like it, but it works.

Problems:
- Can't bind to the `on_channel` events from dashboardview.py for reasons unknown. So I have to have the gauge class call the settings class to save its configuration. 
- The dashboard view pulls the gauge config from the settings object because the gauge class isn't passed a settings object when constructed because Kivy is creating it. This is annoying. Alternative is for the gauge class to listen for `on_settings` and when the settings object is set, pull the saved config. But this will cause the gauge to draw twice (once empty and once if it has a saved config, which is dumb).

No tests yet as I want to wait until this is improved.

Comments appreciated!
so we can't always rely on 'settings' being set here, b/c kivy is creating it...