Add a way to check a value is eql to someone

`expect(x).to.be.in([x1, x2, x3])` will be true if 'expect(x).to.eql(x1)` or x2 or x3.

The idea is to use with any object. Example:

``` js
expect(['encode', 'utf8']).be.in([ ['encode','utf8'], ['overwrite', false] ])
```

I couldn't use `expect([x1, x2, x3]).to.include(x)` because it uses _indexOf_ doesn't work for objects (because implies ===).