Make farming more flexible

The goal of this branch is to make the farming mod more flexible and easier to extend. List of changes:

* New hoes (by @BlockMen)
* API {farming.register_hoe(), farming.register_plant()}
* Fertilities -> Plant only grow on soil with a fitting fertility, e.g. Wheat only grows on grassland, while cotton grows in deserts and grassland)
* New soil: Desert Sand
* Place seeds instaed of plants
![screenshot_9086367](https://cloud.githubusercontent.com/assets/2469742/2756682/5ea73794-c97a-11e3-8d40-fa83c157a418.png)


It still needs some testing and the API documentation isn't finished yet. If something is missing or if you think something isn't ok to merge in, pls tell me. I'll change it :)

(For some reason, git added 2 commits (9fefd3f, 9bd7272) which shouldn't be in this pull request, idk how they come here)