Fix syscall.Kill not support windows

[scbizu]

#2 #18

[coveralls]

Coverage decreased (-0.7%) to 97.426% when pulling f5b6258 on scbizu:master into 36217f0 on go-cmd:master.

[scbizu]

I think it is better to add the cross-platform CI script to recheck if this project supports all platform or not.If not ,and the project will not do, It's better to note it in the README XD

[daniel-nichter]

See #24

[daniel-nichter]

And thank you for the PR :-)