Added formatting for Error objects

Printing an error object from node.js generates an unhelpful `{}` response.

~~~
var e = new Error("Something went bad");
expect(e).to.be(null); // Prints Error: expected {} to equal null
~~~

Passing the response through toString gives a nicer result:

~~~
Prints Error: expected [Error: Something went wrong] to equal null
~~~
Merge pull request #52 from mikeando/master

Addition of formatting for Error objects.