Fixes #3352 - interning error on empty kays
Merge pull request #44 from mbacovsky/3352_interning

Fixes #3352 - interning error on empty kays
Merge pull request #45 from tstrachota/docs

developer docs - first part with howto for commands
several fixes for #2916

- extended maximal cell width in tabular output
- fix for wrong parameter name in exception message
- identifier options are not required when no identifiers have been set