Adding data attribute to all elements

Adding HTML5 data-* attribute to HTMLElement (all ements) as per W3C draft specification (http://www.w3.org/TR/2011/WD-html5-20110525/elements.html#custom-data-attribute).

Usage: 
Element.Data("name", "value") - converted to ... data-name="value" ...

Data attribute name is added to "data-" prefix in lowercase format (not necessary - any non-lowercase names are converted by HTML parser).

Data attribute with empty name is not added to element.