use implicit conversion to RichFile for conciseness
* support for sections under an appendix.
* support for literal, ulink, link, note element.
use a separated temporary directory for each file
add icons, which are of public domain. see http://tango.freedesktop.org/Tango_Icon_Library.
* support for ulink element
* partial support for note element. (only workable for the one with a title element)
support for tables with a title.