Add missing sys import
Fix user having to login after first token refresh

Fix issue where the get_cached_token method returns None instead of the
cached token object, meaning that users have to login again when the
initial token expires. The problem has two aspects:

- The first time the user calls get_cached_token after token
  expiration, the token is refreshed but the "new_token_info" object is
  not returned.

- The second time the user calls get_cached_token after token
  expiration, the scope check fails, because the refresh_access_token
  method didn't include the "scope" key when it cached the new
  token_info results.
Merge pull request #19 from mattduck/fix-auth-token-refreshing

Fix user having to log in after token refresh