Make sure a text rendering of a facts timeinfo is consistent.

[elbenfreund]

Fact.__str__ uses a %d-%m-%Y %H:%M datetime format. Parsing a raw fact however expects
%Y-%m-%d %H:%M. Whilst those formats do not strictly need to be identical it seems highly advisable to avoid confusion and mistakes.

[elbenfreund]

For now we should lettle for the ISO representation. If there is ever a clear preference by the community this may be changed later on. Or we may add a separate config flag alltogether..