Org reader:  make tildes create inline code.

Closes #1345.  Also relabeled 'code' and 'verbatim' parsers
to accord with the org-mode manual.

I'm not sure what the distinction between code and verbatim
is supposed to be, but I'm pretty sure both should be represented
as Code inlines in pandoc.  The previous behavior resulted in the
text not appearing in any output format.