Fix and document default export script (manifestexporter after sitewalker)

[tobiasherp]

The pipeline of the default export script (quintagroup/transmogrifier/export.cfg) contains a sitewalker section followed by a manifestexporter. This is at least misleading (if not worse): the items which are created by the sitewalker are not forwarded by the manifestexporter, and thus thrown away.

This makes the usage unnecessarily difficult. (We don't have documentation and/or useful docstrings in the code, either.)

We should have a working, documented pair of scripts here, which tells about the most important options. One could e.g. simply uncomment the start-path option of the sitewalker section (unless a complete export is intended), and then use the export step; the import script should work equally easy, apart from obviously necessary specifications (which need to be present at least as a comment).

Configuration files of Apache http server, MySQL and PostgreSQL databases etc. (and many more) come nicely documented, with a lot of comments which give valuable hints. We should follow their example.