Add forge block support for Bukkit's Material.isBlock method.

The PlayerInteractEvent will now return the correct value when using its
isBlockInHand() method as well as any other methods that query Material's
isBlock() method.

Before this patch, any plugin attempting to query the Material's isBlock
method would get a false return since it was only checking for id's < 256.
To fix this issue, I now pass a flag when registering bukkit Material's in
the Block constructor.