(Preliminary) pull req - settings instead virtual_midnight

Hallo Marius.

I'm sending 2 preliminary pull requests. So you can see what I'm working on.

In this pull request I'm using settings instead of virtual_midnight. So full settings are available in TimeLog and TimeWindow classes. Other parameters are not altered.

What do you think about it. If there is still many errors like PEP8/formatting, ignore them for now. Is the idea better than in later pull request?