New --unified-diff flag makes expected/actual easier to read without colors

As suggested in this issue: #702

For example, take this test:

```
var a = { name: 'joe',  age: 30, address: {city: 'new york', country: 'us' }};
var b = { name: 'joel', age: 30, address: {city: 'new york', country: 'usa'}};
a.should.eql(b);
```

`mocha`

![inline](http://dl.dropboxusercontent.com/u/11407680/mocha/diff-inline-color.png)

`mocha -C`  (or looking at text-only logs)

![inline no color](http://dl.dropboxusercontent.com/u/11407680/mocha/diff-inline-nocolor.png)

`mocha --unified-diff`

![unified diff](http://dl.dropboxusercontent.com/u/11407680/mocha/diff-unified-color.png)

`mocha --unified-diff -C`  (or looking at text-only logs)

![unified diff no color](http://dl.dropboxusercontent.com/u/11407680/mocha/diff-unified-nocolor.png)

It's my first contrib to Mocha, so another pair of eyes would be helpful.
But hopefully this will make troubleshooting tests easier!