Y.ArrayAssert.hasNoDuplicates()

Hi,

I found myself testing for arrays not having duplicates quite a lot.

Here is a simple example:

```javascript
"should not have duplicates": function () {
  var arr = MyApp.getUsers();
  Y.Assert.isTrue(arr.length == Y.Array.dedupe(arr).length);
}
```

I thought it would make more sense if the test read like this instead:

```javascript
"should not have duplicates": function () {
  var arr = MyApp.getUsers();
  Y.ArrayAssert.hasNoDuplicates(arr);
}
```

Hope you find this valuable.

Thanks!