Extend Android app to support ZKAPAuthorizer-enabled servers

The deliverable is as above with the following additions. The hard-coded Grid configuration may include storage servers which only speak the ZKAPAuthorizer variation of the storage protocol. The application can speak to these servers to accomplish all of the above download tasks.

1. Implement ZKAPAuthorizer economic protocol (read-only) for Great Black Swamp Haskell client library
2. Deploy Great Black Swamp-enabled Tahoe-LAFS storage servers
   1. Add support for economic plugins to Tahoe-LAFS Great Black Swamp implementation
   2. Port ZKAPAuthorizer economic plugin to Great Black Swamp
   3. Release Tahoe-LAFS with the above
   4. Update PrivateStorageio with new Tahoe-LAFS release