A default config.yml file is now always created whenever the plugin is loaded
and there isn't one yet, to cut down on confusion and draw attention to the
configuration possibilities.
Added new option to restrict power blocks (the frame blocks through which the
redstone power is delivered to the portcullis) to certain types of block. This
achieves a few goals:
Make it more difficult to build portculisses by having to use scarce
resources in building them.
Make it easier to use a common material for the portcullis itself, since the
chance of bits of wall flying off accidentally because there is redstone
nearby is smaller if the power block has to be a rare block.
Slightly improve the performance of the plugin.
Added new option to disallow "floating" portcullises, stopping people from
sending portcullises off into the blue sky either by accident (because they
forgot to build a lintel above it) or on purpose.
