Fix missing Material registration on non-item blocks (flax, etc). Closes #172, #55.

The Bukkit API provides a Material enum for plugins to represent an item/block type.
In most cases, new blocks added by mods are added to this enum in the Item constructor.
However, a handful of modded blocks only create a Block, without a corresponding Item:
examples include RedPower2's flax (BlockCustomCrops), IC2 resin holes, Forestry saplings.
This commit adds the addMaterial() call to the Block constructor, ensuring these
blocks always register a Material (duplicate registrations are harmlessly ignored).
Fixes NullPointerExceptions in PlayerInteractEvent etc. when plugins interact with
these modded blocks.