EDBTC修正

取區塊最大值及最小值的部分有誤，應修正為存取原輸入影像的區塊最大最小灰階值。
Merge pull request #41 from B9907012/master

EDBTC修正
[#40] bug fix

now the types of start_clock and cumulative_clock have been changed to
"unsigned long long int" from "unsigned int" for a wider value range.
add cvt_() function
give a parameter 'offset' a default value
Merge branch 'master' of https://github.com/yunfuliu/pixkit