Added HTML support for Button labels 

Currently [`Y.Button`](http://yuilibrary.com/yui/docs/button/) widgets do not support HTML labels.  This is an issue because is isn't uncommon to use nested HTML structures inside of a `<button>` element.  For example...

```
<button>
    <p>I Am A</p>
    <p>Button</p>
</button>

<button>
    <div>Image Button</div>
    <img src="path/to/image.jpg">
</button>
```

To fix this issue, this pull request introduces a `labelHTML` ATTR, and relegates the `label` ATTR as secure sugar around `labelHTML`.

A separate attribute is required for the following reasons:
  1. To not introduce security vulnerabilities to existing implementations who use the `label` ATTR
  2. To convey the insecurity of this attribute by including "HTML" in the name

**Additional notes**
 * Using `@config` instead of `@attribute` in ButtonCore documentation because `@attribute` will include the `Change` events to the API documentation, which ButtonCore (derived from AttributeCore) does not fire.
 * Fixed unrelated breakage in Selleck tests

**Test Results**

	✓ Agent completed: Internet Explorer (9.0) / Windows 7
	✓ Agent completed: Chrome (29.0.1547.65) / Mac OS
	✓ Agent completed: Safari (6.0.5) / Mac OS
	✓ Agent completed: Internet Explorer (10.0) / Windows 8
	✓ Agent completed: Internet Explorer (8.0) / Windows XP
	✓ Agent completed: Internet Explorer (6.0) / Windows XP
	✓ Agent completed: Internet Explorer (7.0) / Windows XP
	✓ Agent completed: Firefox (23.0) / Mac OS
	✓ Agent completed: Safari (4.0) / Linux  (Android 2.3.4)
	✓ Agent completed: Safari (4.0) / Linux  (Android 3.0.1)

	✓ 352 tests passed! (1 minutes, 2 seconds)