Add SoyMsgExtractor.jar to npm package

I've recently added i18n options to soynode (https://github.com/Medium/soynode/pull/26), which works fine if all messages to be translated have already been extracted to a xlf file. To actually extract them people need to use the SoyMsgExtractor.jar though, so I think it'd be nice to have it in the closure-templates npm package as well so that people that already use the compiler also have access to the extractor here.
This pull request is adding it then, let me know if you agree closure-templates should have this.