Workaround (Not Fix) for CraftBlockState Cast Ex.

This is a workaround and not a fix to the following cast exception introduced in Bukkit 1.4.5 API regarding Block.getState().

Until an official fix is created, I've simply bypassed the wall sign blocks from being created, so that users of this plugin may continue to run their 1.4.6 servers without cast exceptions and signs all over the place with a modified build of Population Density 3.6.2.

    java.lang.ClassCastException: org.bukkit.craftbukkit.block.CraftBlockState cannot be cast to org.bukkit.block.Sign