Implement an anonymous credential system for BridgeDB's Social Distributor

BridgeDB's Social Distrubtor is going to need anonymous credentials if it is to safely gather information on bridge usage and/or blockage (used to determine the users' good/bad behaviour). This is briefly touched upon in §5.3.1 of the rBridge paper. I propose implementing and using Camenisch-Lysanskaya 2008 (CL08) credentials, as described in this paper.