export to graphviz dot format

[mpollmeier]

Leaving out the importer for now - mostly because dot is untyped... we could generate 'importFromString' methods for each property in the odb-codegen in the future...

Note: GraphML doesn't natively support list property types, so we fake it by encoding it as a ; delimited string.

Random links:

• https://en.wikipedia.org/wiki/DOT_(graph_description_language)
• https://www.graphviz.org/doc/info/lang.html
• http://magjac.com/graphviz-visual-editor/