do not use literal number, take from info

The headline-levels, which define the maxmum level of pukiwiki header,
are in the options-alist. the alist is passed to all handlers by the
info variable.

Take it from the alist instead of having the literal number "3" in the
code.
Sync with new org-export-to-file and org-export-to-file

As of 6f55864f20 (after 8.0.7 and before 8.1) in org-mode,
org-export-to-file and org-export-to-file changed its argument.  New
functions takes async as the first optional argument, which enables
backends to remove its own async handling.
calculate list item depth without hack

Calculate the list items depth as told by Nicolas[1].  All of my
stupid hack is removed.

[1]: http://article.gmane.org/gmane.emacs.orgmode/70987
Add example block support

Add org's example block support.  Convert the example block as
pukiwiki's preformatted block.