set auth_token to a generated UUID if AUTH_TOKEN not passed in ENV

Fixes issue #191. If an `AUTH_TOKEN` ENV variable is not present, uses `SecureRandom.uuid` to generate a UUID to use as the token.

`AUTH_TOKEN='foobarbaz' dashing start` →` settings.auth_token == 'foobarbaz'`
`dashing start` → `settings.auth_token == 'some-random-uuid-string'`