Use sensible default for success and failure URIs

[pledbrook]

At the moment, if you don't specify successUri and failureUri settings for a provider, you just end up with an exception. THe plugin should really use a sensible default if those settings haven't been provided.

I originally thought successUri should be '/' by default, but then that leaves a big question mark over the default for the failure URI. Perhaps the defaults should be /oauth/success and /oauth/failure? Then the controller could provide default action implementations.