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
Fixed #166 Drop message deserialising makes strange things
Small cosmetic changes
Removed usage of model class Contacts from drop interface

According to the model-view-controller concept, the Contacts class
should not be exposed in the drop interface.
Instead, a collection of "Contact"s is used.
Merge pull request #151 from cburkert/fix_mvc_incompliant_usage_of_contacts

Removed usage of model class Contacts from drop interface.
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.
Merge pull request #169 from k-c13/drop-timestamp

Fixed #166 Drop message deserialising makes strange things
Added link to documentation
Added getKeyIdentifier and getReadableKeyIdentifier to QblPublicKey
Merge pull request #180 from L-Henke/keyIdentifier

Added getKeyIdentifier and getReadableKeyIdentifier to QblPublicKey
Added DropURL class
Removed checks and tests of drop url syntax form DropServer

These tests are specific to URL fully qualifying a drop instead
of a drop server.
Therefore, the checks and tests have been moved to new drop url class.
Adapted code to use DropURL class