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 must be overridden in a subclass or overrides the superclass to prevent something?
If first option, why not make it abstract? Otherwise it should be more clearly state what is intended.

There a occurrences of this in other classes.