Added testling-ci integration. Fixes #124

In this PR, I have adjusted the `package.json` and `test/browser/index.html` to use a common set of JS files such that we can hook into [testling-ci][testling-ci].

The advantage of using [testling-ci][testling-ci] is that it can test against a large number of browsers and presents you with a badge for each commit.

[testling-ci]: http://ci.testling.com/

There will be one additional step taken **before** merging this PR; add the [testling-ci GitHub hook][hook] to chaijs/chai.

[hook]: http://ci.testling.com/#hook

The badge for my forks looks like:
![testling-ci results](https://ci.testling.com/twolfson/chai.png)
https://ci.testling.com/twolfson/chai