Use ox-pandoc for org export

Allows use of pandoc-citeproc. I added pandoc citation style to reftex
citation as well (requires that reftex see the \bibliography latex
command somewhere in the file, of course. For info on ox-pandoc, see
https://github.com/kawabata/ox-pandoc. Some org file settings that are
useful:

BIBLIOGRAPHY:
PANDOC_OPTIONS:
PANDOC_METADATA:
PANDOC_VARIABLES: