build: Clean up npmignore

* npm specific stuff like node_modules and npm-debug.log don't
  need to be listed. They are ignored by default:
  https://www.npmjs.org/doc/misc/npm-developers.html

* Add dist/.jshintrc to npmignore so that it won't be published
  in the tarball and cleanly copy dist/ from the npm repository.

* Remove redundant gitignore file in dist/. This existed to ensure the
  directory was maintained by Git (Git doesn't support empty directories,
  it would've simply not been committed). However there is a jshint file
  there now. 

Change-Id: I9651a1f9641253bd176481ef0515f87385758342