added redirectUri parameter to GetAuthorizationUrl

By specifying the ```return_url``` parameter to ```/authorize```, the user will automatically be redirected back to the calling web application after authorizing access.

Instead of creating an overloaded method for this, I thought about including ```redirectUri``` as an optional parameter, but in the original implementation of ```GetAuthorizationUrl```, you're using ```/connect``` instead of ```/authorize```, but I couldn't find any documentation on ```/connect```. Perhaps this is an old endpoint that is only left open for backwards compatibility?