Tests with timestamps

I made one change to the latest command, such that datanommer-latest --timesince would be more readable with pretty_dumps. I also provided tests for more dump and latest arguments: 
datanommer-dump --since
datanommer-dump --before 
datanommer-latest --timestamp
dataommer-latest --timestamp --human
datanommer-latest --timesince
datanommer-latest --timesince --human
Personally, I feel a lot more comfortable with datetime and timestamps now :)