Tweak htmlfontify's generated output

Tweak htmlfontify's generated output

* lisp/htmlfontify.el (hfy-default-header): declare the character
encoding for the generated HTML file.  This helps browsers display
UTF-8 characters like the copyright symbol correctly.
(hfy-sprintf-stylesheet): apply the default style to all of the text
consistently, so that unstyled bits (which are not wrapped in span
tags and are directly part of the surrounding pre tag's "innerHTML")
have a look consistent with the rest of the document.