Add Y.ArraySort.naturalCompare()

# Y.ArraySort.naturalCompare()

This change adds a static `Y.ArraySort.naturalCompare()` method, which performs a natural-order comparison of two strings, two numbers, or a number and a string. This ensures that a value like 'foo2' is sorted before 'foo10', whereas a standard ASCII sort would sort 'foo10' first.

See the API docs and code comments for more details.

## Example

```js
var items = ['item10', 'item2', 'item1', 10, '1', 2];

items.sort(Y.ArraySort.naturalCompare);
console.log(items); // => ['1', 2, 10, 'item1', 'item2', 'item10']
```

## Test Results

All tests pass on a representative subset of supported browsers:

```
✓ Testing started on Firefox (20.0) / Mac OS, Safari (6.0.4) / Mac OS, Chrome (26.0.1410.65) / Mac OS, Internet Explorer (10.0) / Windows 8, Safari (6.0) / iOS 6.1.4, Internet Explorer (6.0) / Windows XP
✓ Agent completed: Safari (6.0.4) / Mac OS
✓ Agent completed: Chrome (26.0.1410.65) / Mac OS
✓ Agent completed: Firefox (20.0) / Mac OS
✓ Agent completed: Internet Explorer (10.0) / Windows 8
✓ Agent completed: Safari (6.0) / iOS 6.1.4
✓ Agent completed: Internet Explorer (6.0) / Windows XP
✓ 48 tests passed! (6 seconds)
```