Added support for CommonJS/browserify.

Per our conversation on IRC, a PR for CommonJS/browserify support. :beers: 

#259
#253
#206
I think anyone using browserify will be fine with https://github.com/umdjs/umd/blob/master/returnExportsGlobal.js which seems a little cleaner and more straightforward. What would you like to support: CommonJS or browserify? From the issues, it seems as though most people are interested in browserify.