Adding Clouds plugin for world generators

This adds a plugin for world generators that computes clouds (at 50% coverage) based on Perlin noise. It works as-is, but there are certainly many things that can be improved such as the height of the cloud layer (currently 127 as in Minecraft).

NOTE: This also contains the fixes from #1 to make the code compile properly.

Requires https://github.com/MovingBlocks/Terasology/pull/1442 to work.