Added support for UMD's returnExports (https://github.com/umdjs/umd)

This makes it possible to use moment.range with RequireJS, compiles
RequireJS as well as Node.

A possible improvement would be to introduce a grunt build step and use
[grunt-umd](https://github.com/alexlawrence/grunt-umd).
Merge pull request #1 from frekw/added-umd-support

Added support for UMD's returnExports (https://github.com/umdjs/umd)