Naming in drop message was wrong
Fixes singular/plural naming inconsistencies in config classes

This changes some names of collection members and collection returning
getters to indicate the plural.

Closes #103
Small cosmetic changes
Update class diagrams
Merge pull request #178 from k-c13/update-diagram

Update class diagrams
Merge pull request #167 from cburkert/fix_naming_inconsistencies

Fixed #103 singular/plural naming inconsistencies in config classes
Merge pull request #163 from k-c13/drop-message-naming

Fixed Qabel/qabel-doc#69 - Drop message fields were named incorrectly in the implementation.