You can't extend expect.js outside expect.js

It turns out that you can't sensibly extend expect.js outside expect.js. expect.js's `i` function (used for inspecting objects) is not exported.

Many of the functions in `test/test-extensions.js` don't actually work at all - they should call `this.assert` instead of returning a boolean. This applies to #362 and #368. These functions pass when they should fail.

I'm working on it now.