Dithering algorithm implemented for IMSH

Today I implemented Dithering algorithms for IMSH (IMage SHell) - see Project WiKi - Dithering. They are: general purpose (see matrixes in imsh.scm library) and special purpose: Atkinson, Floyd-Steinberg, Ordered, Random, General Sierra, General Sierra Two Rows, General Sierra Low, General Floyd-Steinberg, General False Floyd-Steinberg, General Jarvis, General Stucki, General Atkinson, General Burkes.