Added Xbox One support, fixed some bugs, and restructured code.

* The Settings class has been rewritten, wrapping both local storage and sync storage into one function call, this reduces clutter and seals the portal to Callback Hell elsewhere in the code.
* All relevant code in the background, popup, and settings pages has been updated to use the new Settings class.
* Modified the Api class in the spirit of the new Settings class, although not a complete rewrite it combines both alerts and invasions to reduce clutter elsewhere.
* The Api.fetch() method now requires the platform be passed as the first argument, rather than using a class property, this would sometimes cause a bug where the Api class would not load data from the new platform when the setting is altered from the options page.
* Removed a lot of dead code in comment blocks.
* Made the code a little bit more DRY.