Add browserify support.

I think we've juggled around our UMD config enough times to know that if possible, it would be best if it just weren't a part of our source anymore.

Browserify offers the great `standalone` config option that [generates a UMD shim for you](http://www.forbeslindesay.co.uk/post/46324645400/standalone-browserify-builds). If we use it, we gain:

* Simplier src, which can be required directly from the node shim, without UMD
* A build compatible with AMD/ES6/Browser/Browserify

If we go this route, we unfortunately introduce a build step. I think it's worth it to clean up the hacks in our src and to allow us to offer minified versions, etc.

If you guys are okay with the build step, let's talk about:

* Is there a simpler way to do lines 7-10 of src/backbone.layoutmanager.js?
* Should we do a `dist/` folder instead with an uglified version and link both from the README for browser users?