Common.JS Support

Use case: using browserify to write client-side tests in  Common.JS format. This patch adds support for `sinon.useFakeXMLHttpRequest` and `sinon.fakeServer`.

Changes in `fake_server.js` involve mostly a deeper indentation level and proper wrapping.

I ran `npm test` and they pass. Unfortunately, wasn't able to install `gem install juicer` to test in the browser :(

I'm writing a book where I'd like to use `sinon` to write tests for the browser using Common.JS through browserify, this patch would enable that. So I'd really love for you to merge this thing in!