Provide autorefresh of OAuth tokens

Currently, QtNetworkAuth requires you to manually check if the access token has expired and needs refreshed. I propose that NetworkAuth should automatically perform refreshing of OAuth access tokens:

1. When the access token is retrieved, NetworkAuth checks to see if a refresh token was provided. If so, it stores it for later use; otherwise, it assumes that refreshing the token is not needed.
2. NetworkAuth attempts to extract an access token lifetime from the access token response body. If it can be retrieved, NetworkAuth starts a single-shot QTimer that will fire about 10 seconds before the access token expires. The timer's payload simply calls refreshAccessToken() on the OAuth object.