Use .npmignore to keep test files out of distributable

[davidmsibley]

https://docs.npmjs.com/misc/developers#keeping-files-out-of-your-package

A couple times in the past we've run into issues trying to test frame apps, because app-framework's spec files and karma configuration is present in the frame app. It finally clicked that we may only need to add a couple of entries in .npmignore to fix that. If that doesn't work, worst case we'd need to remove the files in Maven. We could test the change with widget-creator to see if it simplifies the config.

[ChristianMurphy]

another neat trick.
.npmignore is a blacklist of files to ignore.
package.json can include a files attribute that is a whilelist of files to include.
https://docs.npmjs.com/files/package.json#files