Added extra convenience methods, separated convenience methods into ExtensionsForHtmlTag class

Extra convenience methods: Value, Href
Extra toggle methods: Readonly
Extra style methods: Color, Padding, Margin, TextAlign, Border

All convenience methods are no longer part of the HtmlTag class but now
reside in the 'ExtensionsForHtmlTag' class. This does not impact any
namespaces and should therefore not break existing code.