[Petal] [REQ] Escape HTML entities

[Jean-Michel Hiver]

> - Case sensitivity, use lower case for XHTML 
>   http://www.w3.org/TR/xhtml1/#h-4.12

There is no need to encode anything else than < > " and &
in the output. I am not intending to have an output full of   and
©


> - The named character reference ' (the apostrophe, U+0027) was
>   introduced in XML 1.0 but does not appear in HTML. 
>   http://www.w3.org/TR/xhtml1/#C_16

Doesn't matter - XML::Parser will expand it anyway since it's XML.