Use --test for interface documentation
First version of documentation for messages

(Add "CLASS Message" after an interface name to classify it as a message interface) @stefjoosten: expression and definition will probably not both fit in the table, so we have to make a choice or think of some other presentation.
Only create messages chapter if there are message interfaces
Show interface name in message table in bold
Merge pull request #17 from AmpersandTarski/document-message-interfaces

Generate documentation for Message interfaces

Still requires explanatory text and better table layout (last two columns in message table are too wide, so only one is partially shown.)