能基本工作，已知问题，Del运算结果的最后一位出现问题
Fixed
Merge pull request #1 from linsk/master

kaka
Update README.md
多次运算以后负负得正问题...
merge readme changed
连续运算bug
num2 = 0 fixed
解决连续计算后Del最后一位出NUll
update Readme
Merge branch 'master' of https://github.com/linsk/calculator into dev