NickHodges' THTMLWriter ported

[Leledumbo]

I've ported NickHodges' THTMLWriter units. The main conversion process was to replace TStringBuilder with plain String everywhere. However, the example fails to work. I can't manage to close the head tag, if I do so, exception would be raised (see the commented line in example dpr). Seems like I delete something in the porting process. Please help me fixing this so the unit can be used perfectly.