Remove default value from all redirectUri parameters

[bocops]

We currently use urn:ietf:wg:oauth:2.0:oob as default parameter for a redirect URI both when first registering an app with an instance (AppMethods.createApp()) and when performing OAuth (multiple methods in OAuthMethods).

This URI is a special case in that it does not actually redirect anywhere (most likely to a URL that will be intercepted by the app starting the OAuth flow), but will just display a valid code in a browser window. This might be useful in some development contexts, but makes no sense for production code.

I already argued in #68 that we shouldn't use the URI in any sample code. I now think we should not use this URI as default parameter in any of the methods mentioned above, either, but instead make the library user explicitly define a redirect URI in every case. Keeping it as a default parameter might lead to hard-to-debug situations where an actually sensible parameter is omitted and authentication fails as a result of this.

This would not remove the possibility of using this URI, its use would just need to be made explicit - we could even define it as a constant for easy access, if considered useful.

[andregasser]

@bocops Lets change this.