add .npmignore

During npm publish, certain files and directories can be ignored from the package.
See https://npmjs.org/doc/developers.html#Keeping-files-out-of-your-package.
This reduces the package size.
Package consumers need only the minimum files necessary to execute the package functionality.

**Keeping**
* AUTHORS
* CHANGELOG
* CONTRIBUTING.md
* LICENSE-MIT

**Ignoring**
* docs
* test
* .jshintrc
* .travis.yml
* Gruntfile.js