WPSNR function imported.

From
<http://www.mathworks.it/matlabcentral/fileexchange/3675-wpsnr/content/wpsnr.m>.
Putting some constranits for the CTM competition.

* Fix corner-case image put in spread_spectrum_decode(),
* Setting handcoded some constranits to use in the ctf, as image sizes - no time
  for shitty global variables.
* Adding {embedding,detection}_biscotto as requested from CTM rules.