Decreasing test precision

Tests fail on https://cpantesters.org/. I think this is because it took a bit
more time to run the commands on their servers. I hope this commit will fix
that problem.
New method is_completed()
Small additions to POD that describe unobvious things
get_elapsed_seconds() always return process run time

Before this commit get_elapsed_seconds() always returned the number or seconds
from the process start moment. After the process is complete this behaviour is
wrong.

https://github.com/bessarabov/Time-ETA/issues/1
Main test file is completly rewritten
Storing process end time in the serializied string
A small addition to POD