BibCatalog: fix abstract class instantiation
- Changes the return value of importing bibcatalog_system from bibcatalog.py to None when no ticket system is configured. (fixes #780)
- Adds a check to BibCatalog clients BibEdit and BibHarvest when using bibcatalog_system to see if any ticket system is configured.
- Fixes the abstract class BibCatalogSystem to raise NotImplementedError on instantiation and use of the class.
- Updates the BibCatalog hacking guide.