stddoclet: Add CSS style for setting header/footer to be italic

The header/footer are currently hard-coded to <EM> so that
the following produces an italic header:

-header "Java 2 Platform Std. Ed. v1.4.0"

It would be cleaner to do this in the style sheet, so
it could be overridden in the style sheet rather than
in the -header argument.

That is, rather than:

-header "</EM>Java 2 Platform Std. Ed. v1.4.0<EM>"

you could do:

.HeaderText { font-style:italic; font-size:80%; }

and override it by changing "italic" to "normal".