[O] ox-md conforming pandoc-markdown

[Michel Kuhlmann]

Hi,
I often convert files with `pandoc`. Currently there is no
pandoc-org-mode-reader, so I'm using markdown as an intermediate step.

I'm aware of <https://github.com/robtillotson/org-pandoc.git>; this
also uses `ox-md`, so the issue applies also there.

I found these issues inconvenient:

- Listings create blank-lines in between
    + Currently
                        - uno
       - uno
       - dos    ->      - dos
       - tres
                        - tres
    + Desired
       - uno            - uno
       - dos    ->      - dos
       - tres           - tres

- Formulas are getting surrounded by \(  \); 
    + Currently
        $Q_{max}$  -> \(Q_{max}\)
    + Desired
        $Q_{max}$  -> $Q_{max}$

- customisable meta-data export (currently nothing exported) of, say,

    #+TITLE: myTitle
    #+KEYWORDS: cat1 cat2
    #+AUTHOR: meMyself
    #+DATE : 2014-01-21
    #+OPTIONS: toc:t

    + Desired
        * meta-yaml-block

            ---
            title:  myTitle
            author: meMyself
            catgories: cat1 cat2
            date: 2014-01-21
            toc: toc:t
            ...

        * pandoc header

            % myTitle
            % meMyself
            % 2014-01-21

Kind regards,
Michel