Extend Android app to support arbitrary N/k CHK capabilities

The deliverable is as above with the following additions. The application can download the associated data from N Tahoe-LAFS servers speaking the "anonymous" Great Black Swamp. The Tahoe-LAFS servers is located via hard-coded Grid configuration. The same servers are selected as the Python Tahoe-LAFS implementation would select. If the associated data is plain text then it can display it in some basic way.

1. Build Tahoe-LAFS server selection algorithm compatibility test suite
2. Implement Tahoe-LAFS server selection algorithm in Haskell