Remove unnecessary files from npm package

NPM users don't need browserified bundle, tests or Makefiles.

```
package/package.json
package/LICENSE
package/index.js
package/selectn.js
package/Readme.md
package/Makefile
package/component.json
package/examples/stats/package.json
package/examples/stats/index.js
package/examples/stats/stats.json
package/.travis.yml
package/History.md
package/.editorconfig
package/test/index.js
package/test/index.browser.html
(10351 bytes total)
```

This PR shrinks the package by discarding everything except these files:

```
package/package.json
package/index.js
package/Readme.md
(3130 bytes total)
```

The decision of exactly what files should be included is somewhat arbitrary, but I think removing unnecessary parts is worth considering anyway.
`package.json` and `Readme.md` are included [by default](https://docs.npmjs.com/misc/developers#keeping-files-out-of-your-package).

Added LICENSE in a latest commit.

```bash
$ npm pack
selectn-0.9.6.tgz
$ tar tf selectn-0.9.6.tgz
package/package.json
package/LICENSE
package/index.js
package/Readme.md
```