Fix #182. Fix #183. Refactoring of saplings code.

Now there are 3 categories: Flora Sapling, Rare Sapling, and Nether Sapling.

I worked hard to clarify the sapling code and get rid of some more subtle bugs. 
I tested by planting each type of sapling on various soils, letting them grow naturally, and using bonemeal on them.
Checked that: 
 * Sapling names stay the same
 * textures are correct
 * tooltip is correct
 * tree it grows into is correct
 * can't be placed on the wrong soil
 * can be placed on soil that supports it
 * bonemeal fails if growth conditions are not met (redwood)
 * saplings show up correctly in creative inventory and NEI

This will break some saplings in existing worlds, but doesn't touch trees or leaves. Since 1.7.10 is very early and still not in stable packs, I think this is the right time to make this change.

Cheers :bee:
Sounds good, I'll do that.