no CommonJS dependencies if run in browser

I'm wandering if it wouldn't be better to reduce CommonJS dependencies, i.e. require('xmlhttprequest') and require('underscore') when running in a browser.

I'm using this awesome library from within TiddlyWiki, which runs in a browser and also uses kind of CommonJS requires.

P.S. sorry for my first pull request - it was build against a different fork, which used fewer underscore methods.