Use mocha instead of vows.

I took some liberties here based on how I thought this should work. Would love to discuss.

![screen shot 2014-05-15 at 20 34 55](https://cloud.githubusercontent.com/assets/84700/2992448/f2052a9e-dc91-11e3-852f-41b7eed35dc7.png)

This currently introduces a breaking change with the removal of `.next()`. If that's an issue we could add a no-op for that and issue a deprecation warning.

* Remove `.next()` from the API since it was unnecessary.
* Remove `.run()` from the API since it was specific to vows.
* Replace `.discuss()` with `.describe()` and `.it()`.
* Update CoffeeScript.

Related to #7.