Merge pull request #17 from PeterLai9527/master

modified HPSNR function
modify psnr func
add static_cast
Update NADD.cpp
Update pohe.cpp
Update LICENSE.txt
pixkit::enhancement::global::GlobalHistogramEqualization1992
Update NADD.cpp
Update pohe.cpp
bug修正

bug
Update example_MaryKim2008.cpp
Update example_MaryKim2008.cpp
Merge pull request #21 from JieCyun/master

bug修正
test

test
change example
Update example_MaryKim2008.cpp
Update prog.vcxproj
Update prog.vcxproj.filters
Update enhancement.cpp
Update enhancement.cpp