Extracting the n method for BlockStem and BlockCrops into their own superclass method to reduce code duplicity

Before, there was code duplication within the BlockStem and BlockCrops class. While there is still code duplication (that requires testing random), it is reduced. Likewise, we have created a nice TestWorld inner class for simplifying the very complex world object to test it within the context of testing that n method. We have created BlockAgriculture to be a superclass to BlockStem and BlockCrops. BlockAgriculture extends what BlockStem and BlockCrops previously extended, BlockPlant, in order to maintain previous functionality.

This n method is very well tested and has been tested from before the extraction, during the extraction, and after the extraction.
![wax on wax off](https://cloud.githubusercontent.com/assets/5413654/2849419/faccffe6-d0d7-11e3-981a-36af651087b5.jpg)