Add RefreshToken to the WebServer auth response

Uses the same pattern as the existing object fields, so the field will be present but empty if you make a request that does not include 'refresh_token' in the initial call's scope
Merge branch 'CaptureRefreshTokenFromWebServerResponse' of github.com:eli-precisionlender/Force.com-Toolkit-for-NET into eli-precisionlender-CaptureRefreshTokenFromWebServerResponse
Merged and updated code to add token refresh