Added canPlaceMultipart to handle FMP multipart placement.

ForgeMultipart and MCPC+ are now working together to handle permissions
for multiparts. Before a player places a FMP multipart, FMP will call
World.canPlaceMultipart to see if it is allowed to place. MCPC will fire
a BlockPlaceEvent to plugins to check permissions and return the result to
FMP to process. If false, FMP will prevent placement. If true, FMP will place
the part.

Thanks to ChickenBones for adding support to make this happen.