ETA feature

Hi Sebastian,

I wanted `ProgPercent` to display ETA (Estimated Time of Arrival/Completion) next to the elapsed value. I took the opportunity to make the code slightly more PEP8 compliant and made a couple of slight refactorings as well.

ProgPercent looks like this with the ETA:
```[ 30 %]   elapsed: 3.000 sec - eta: 7.000 sec```

This pull request is not ready to merge yet. If you like this idea, then I'll update the documentation and push the changes to this pull request. I can also change the formatting of the ETA value if you like. If you don't like this idea, then you can act like this pull request ever happened :smiley: