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).

Sorry for the huge whitespace diff.

This fixes https://github.com/gf3/moment-range/issues/23