[WIP] Rewrite documentation in MarkDown

This (still WIP) branch contains all the current PowerDNS documentation (as of 17d953b0aad6c3b03abf84a8c25aeed1c171edb5) in MarkDown format (in the `pdns/docs/markdown`) directory.

To build the website, you'll need [mkdocs](https://github.com/tomchristie/mkdocs), the current HEAD works and can be installed (in e.g. a virtualenv) using `pip install git+https://github.com/tomchristie/mkdocs.git@80c9b2842c8665a1ace189cc6991b97c996703d6`. Additionally, the `pandocfilters` package needs to be installed.

Running `make html-new/index.html` in the `pdns/docs` directory will build the website in `pdns/docs/html-new`.

The documentation still needs a rewrite, this is the first step. Please verify the text of the docs and report issues in this PR. The current version of the docs can be found [here](http://pdns-docs.plexis.eu).