Refresh access token during sensor startup

[unlobito]

This PR closes #2 and partially fulfils #1.

There are three main changes in this PR:

• glow.py is no longer flagged as async capable
• Calls to glow.py are wrapped in hass' async helper
• Sensor setup can detect & recover from expired access tokens

Unfortunately, this still requires the user to restart hass or otherwise reload the component, so there's a bit more work to be done on #1.

[unlobito]

Merging this in now as a beta fix for #1