Add BlockMultiPlaceEvent. Adds BUKKIT-5558

Some blocks, such as beds, doors, or flowers, are actually composed of
multiple blocks when they are placed.  Currently, to detect how many
blocks are actually modified a plugin has to perform various calculations
to determine the directions of relative blocks, many of which are
difficult to perform and can easily return false positives.

This commit adds in a BlockMultiPlaceEvent, which adds in easy support for
accessing all blocks modified by the placement of a block.