Add UMD support

Relating to what's been done here:

https://github.com/madebymany/eventable/pull/3 (https://github.com/umdjs/umd)

We should be providing this support for all JavaScript things that we're open sourcing.

If all's good with this I'll register it on NPM and we can start using `npm install bigbird` for future projects using Browserify.

/cc @cjbell88 @ninjabiscuit