Add "browser" field for Browserify

Browserify and other CJS builders will look for a "browser" field to use as an alternate "main" when targeting the browser environment.

https://github.com/substack/node-browserify#packagejson

Pointing it to the compat build would prevent issues like #288 happening out of the box, without any extra configuration.
Merge pull request #466 from aearly/patch-1

Add "browser" field in package.json for Browserify. [ci kip]