Added HTML support for Button labels

[derek]

Currently Y.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)

[derek]

Closing this PR and reassigning to dev-3.x.

Submitted: #1163