Support Markdown .md Files as /content Documents

[escv]

Implement a IResourceConverter OSGi Component in content module to support reading in .md files.
The result should be a XHTML plain and simple markup.
The Code can be aligned to CSVResourceConvert. It should be checked if an additional markdown library is required or a GIST/Snippet can be used to implement it fully without 3rdParty dependencies.

Some external libraries do not fully support OSGi bundling.