imgcodecs: Implementation of PAM encoder/decoder

This patch implements the PAM image format as defined at:
http://netpbm.sourceforge.net/doc/pam.html

The PAM format provides a generic means for storing 2 dimensional information.
This is useful for opencv since there are cases where data gets translated into
non standardized formats, which makes it difficult to store and load this information.

modules/imgcodecs/src/loadsave.cpp

@@ -109,6 +109,8 @@ struct ImageCodecInitializer
         /// Attach the GDAL Decoder
         decoders.push_back( makePtr<GdalDecoder>() );
     #endif/*HAVE_GDAL*/
+        decoders.push_back( makePtr<PAMDecoder>() );
+        encoders.push_back( makePtr<PAMEncoder>() );
     }
 
     std::vector<ImageDecoder> decoders;