Attribute group declaration not working correctly

[severn-everett]

A couple of attribute group declarations aren't being translated into attribute fields for some tags, for example:

• The button tag does not incorporate the CausesValidation attribute.
• The param tag does not incorporate the coreAttributeGroup group of attributes.

A different way of translating attribute groups should be implemented wherein attribute groups should consist of two different fields:

• A list of attributes that are declared directly within the attribute group in the html_5.xsd file.
• A list of parent attribute groups that the declared attribute group inherits from.

A benefit of this change will be that the creation of the attribute groups for the SVG tags (for issue #26) will be easier.