Implement #15, add a debug option.

Org-site will generated some index files during publishing process and
delete them by default after publishing. Sometimes you need to keep
these index files to track some code issues, so this debug option
is used to keep the generated index files when necessary.
Implement #10, add function to read properties of org file.

This function is needed by TAGS/CATEGORY support since TAGS/CATEGORY is
one kinds of org file metainfo.