Introducing new sky model: Hosek-Wilkie.

It is new developed model. Gives IMHO more natural sky colors and degree contrast. One flaw comparing to old model is that it does not evaluate colors for directions under horizon. Simply horizon color is expanded to lower hemisphere.
All directions of sun basing on date/position remains the same. Only one new setting is added here: albedo - it is planet reflectivity.

For some time old sky model remains set as default.