Enable chai.includeStack option for more detailed failures

The [`includeStack` option](http://chaijs.com/guide/styles/#configure-section) will make chai prints detailed stack traces on uncaught errors.

![screen shot 2013-12-19 at 10 02 22](https://f.cloud.github.com/assets/41547/1781129/9c8d3ff8-688c-11e3-98fd-0b2f273bddd3.png)