GmailSnooper almost done

Still missing:
- Read email address from DB.
- `bearertoken.access_token` and not `bearertoken.access_token['access_token']`
- Token expiration.
Update a docstring
Fix a issue when a provider has not been added
Refactor BearerToken model: change _access_token to _token_set
GmailSnooper almost done

Still missing:
- Read email address from DB.
- Token expiration.