Migrate to aiohttp, fix update_interval

[bjw-s]

So I went on a bit of a quest it seems... I started out trying to fix #7, because I was seeing some annoying exceptions as well. That led me to replacing requests with aiohttp which in turn led to adopting the DataUpdateCoordinator approach. So there are quite some changes going on:

• Replace requests with aiohttp (see https://github.com/cgarwood/python-fullykiosk/pull/6)
• Implement a custom DataUpdateCoordinator
• Make sure all entities inherit from the CoordinatorEntity so that the update_interval is actually respected
• Make all the service calls async-compatible
• Add the reboot_device service