Release 2.5.0

• Added a reversible data hiding technique based on histogram shifting
  (stegano.rdh), with a stegano-rdh command line tool. Unlike LSB, the
  original cover image can be recovered exactly after the message is
  extracted. Implements the method of Z. Ni, Y.-Q. Shi, N. Ansari and W. Su,
  "Reversible data hiding," IEEE Transactions on Circuits and Systems for
  Video Technology, vol. 16, no. 3, pp. 354-362, 2006
  (doi:10.1109/TCSVT.2006.869964).
  Thanks to Eesh Saxena
  (#71).