Add a setter for NodeList.length

Apparently, Firefox's `Array.prototype.splice` sets `length`. This fails without
a setter, so add one that just returns the current value of `this._length`.
Make jsdom browserifiable and add browser test runner

To run the tests: `node test/browser-runner.js`. The runner accepts the same
options as `test/runner`, as well as some optional arguments that control the
test setup.