gamma, contrast, brightness adjustments
fix comment
update readme
contrast calculation bug fixed
no need to fill a new image with zeros
New: width and height - check positive
AdjustSigmoid function added
math import
upd readme
TIFF and BMP image format support added. Now Mercurial is needed to 'go get' the package
Revert "add Gamma function, refactor color mods"

This reverts commit c59c28a3c2996eb711755688a4d6e50fb06f9d08.
Merge remote-tracking branch 'upstream/master'