UMD-ready Underscore

Underscore.js is now an [UMD] [1] module.

The UMD definition is based on https://github.com/umdjs/umd/blob/master/returnExports.js.

A stub `noConflict` function is provided in Underscore.js to retain API compatibility.
The real `noConflict` method is only defined once if Underscore.js is not loaded as-is in the browser (ie. as an AMD-module or in a Node.js environment).

[1]: https://github.com/umdjs/umd "UMD"