add halftone functions
Merge pull request #15 from b9707137/master

improve twoPass()
merge with upstream
remove descriptions of halftoning methods
modify halftoning functions
add pixkit::Timer
Merge branch 'master' of https://github.com/yunfuliu/pixkit
modified halftone functions
Merge pull request #16 from PeterLai9527/master

edit halftoning functions
improve pohe code
improve description of pixkit-timer.hpp
improve cmake file
modify with env var
modify .cpp
Update README.md
Update README.md
Update MANUAL.md
quality assessment function HPSNR

HPSNR assessment
Merge branch 'master' of https://github.com/yunfuliu/pixkit
modified HPSNR function

modified HPSNR function for error condition