Add license header check for all files

[zymap]

Fixes: #65

[wolfstudy]

LGTM +1

@sijie please review this pull request, thanks.

In here, we treat the check of the license header as a test file, so when we execute go test ./..., we will check all files for the correct license header.

[sijie]

@wolfstudy @zymap we can go with this approach for now. but is there any generic license header check tools in go?

[wolfstudy]

@wolfstudy @zymap we can go with this approach for now. but is there any generic license header check tools in go?

There is currently no universal license header checker in the go community.