Update API for Acacia & Dark Oak. Fixes BUKKIT-4988

When Minecraft 1.7 was released, Acacia and Dark Oak TreeSpecies were
added. While Bukkit added them to TreeSpecies it did not add support in
org.bukkit.material.Tree getSpecies() and setSpecies().

Currently Acacia (0x4) is reported as Oak (0x0) and Dark Oak (0x5) is
reported as Spruce (0x1). This commit fixes that by splitting the
classes that blocks are assigned, giving WOOD/SAPLING/WOOD_STEP all 6
TreeSpecies, and LOG/LOG_2/LEAVES/LEAVES_2 their own classes supporting
the proper TreeSpecies for each.