Add Biome decorator

This is extensible and could help to implement the other things to decorate biomes with variations.
It already contains:
 - tall grass (and ferns but only in some biomes, so will be visible later...)
 - flowers and double flowers
 - sugar canes
 - dead bushes
 - pumpkins
 - melons
 - water lilies
 - cactuses
 - desert wells

![grass and flowers](http://s4.postimg.org/7yeyz9ufh/2014_10_24_21_59_48.png)

![desert well](http://s29.postimg.org/qxr8azf6f/2014_10_24_18_21_17.png)

That was intentional for the tests so can locate it more easily ^^.
I'll remove it.