Update: Operator CHALLENGE command/functionality for tethys

For the git history log:
  Client-side respond/signing program has moved to a public repository
  https://bitbucket.org/aaronmdjones/chal-respond/src

At the request of Alex:
  Migrate previous implementation to a different base64 codec

At the request of Hugo:
  Include space for null terminator when inflating
  Fix a variable type in the encoder