Add db:aggregate() tests for the future option to only aggregate unique values

This tests should FAIL currently as the corresponding functionality ins not yet implemented.
Additionally some comment changes.
Fix of a unit test of the last commit.

Additionally two minor changes.
This build should also FAIL.
improved db:aggregate()

The improvement enables the user to aggregate only distinct values.
Merge branch 'master' of https://github.com/vadi2/mudlet-lua into AggregateDistinct
Merge pull request #5 from keneanung/AggregateDistinct

Aggregate distinct