Fix npm.js to avoid polluting the global namespace

Created by: jesusprubio

Hi, I've opened #15638 (closed) that I'm solving with this PR. I've also added a note to the README to show how to load the library without exporting/overwriting anything in the global namespace. Now we can require it in this way:

var jquery = require('jquery');
require('bootstrap')(jquery);

console.log('Is Bootstrap loaded?');
console.log((typeof jquery().emulateTransitionEnd === 'function'));