Added skystone, skystone block, skystone brick, skystone small brick, certus quartz, certus quartz pillar, chiseled certus quartz and fluix stars, closes #52

To integrate these into the current system, some changes to the background had to be done, especially to the feature handler. It now uses an interface to work against which you can implement to get your own feature handler instead modifying the base one and add several special cases code.
Does this mean it should be overridden in a subclass or is overriding the superclass?
If the first assumptions matches, why not mark it as abstract?