Overriding method should have "virtual"

Fix warnings on OSX
add `nwsnapshot` to package tool
Merge pull request #463 from owenc4a4/tool

add `nwsnapshot` to package tool
Merge pull request #464 from owenc4a4/test

[test] test case for screen shot
Merge pull request #467 from owenc4a4/test

[test] test case for snapshot