Adds stack traces to the failures just like mocha does in the browser.

This adds stack traces to the error messages like this:

```
>> Stacktrace: 
>> (anonymous function)			http://localhost:7002/lib/chai.js:241,
>> include			http://localhost:7002/lib/chai.js:442,
>> (anonymous function)			http://localhost:7002/lib/chai.js:2470,
>> (anonymous function)			http://localhost:7002/spec/views/credentials/new.js:25,
>> (anonymous function)			http://localhost:7002/lib/mocha/mocha.js:3460,
>> (anonymous function)			http://localhost:7002/lib/mocha/mocha.js:3772,
>> (anonymous function)			http://localhost:7002/lib/mocha/mocha.js:3831,
>> next			http://localhost:7002/lib/mocha/mocha.js:3700,
>> (anonymous function)			http://localhost:7002/lib/mocha/mocha.js:3709,
>> next			http://localhost:7002/lib/mocha/mocha.js:3657,
>> (anonymous function)			http://localhost:7002/lib/mocha/mocha.js:3672,
>> done			http://localhost:7002/lib/mocha/mocha.js:3438,
>> (anonymous function)			http://localhost:7002/lib/mocha/mocha.js:3450,
>> (anonymous function)			http://localhost:7002/spec/views/credentials/new.js:16,
>> (anonymous function)			http://localhost:7002/components/jquery/jquery.js:1129,

```

Previously I often found myself running the mocha tests in the browser just to get the stack traces from the errors.