Added category interfaces to BUTTON, INPUT, SELECT, LABEL, and SELECT.

See the HTML 5 spec [Forms page](http://www.w3.org/html/wg/drafts/html/master/forms.html#categories) for more information. To determine in an element has an associated form, it is as easy as:
```csharp
IDomElement e = doc["some selector"].FirstElement();
IFormAssociatedElement f = e as IFormAssociatedElement;

if(f != null)
    ...
```

Also:
- Added `Disabled` property to `IDomObject`, much like `ReadOnly` and `Checked`.
- Added correct implementations of `Disabled` to form fields that can be disabled by a FIELDSET.
- Tests for new product code.