add .npmignore to addon blueprint

Having an `.npmignore` in your addons directory can reduce the size of the packaged and published node module considerably. For example: In case of the [ember-cli-i18n addon](https://github.com/dockyard/ember-cli-i18n), the size of the tgz file build with the `.npmignore` in this PR is only 1.5% of the one build without the `.npmignore`.

I'm not sure if calling `tar` (line 204 in the test) without any checks is such a good idea...The test probably fails on Windows. What do you think about this?

This resolves issue #2640 


@stefanpenner ok I'll do that tomorrow