Add support for custom air blocks (e.g. Railcraft aura) in Bukkit API isEmpty. Closes #1284

Forge allows mods to add custom air blocks, which return true for isAirBlock().
Bukkit has the notion of an 'empty' block (isEmpty() returns true), but it
was hardcoded to only recognize Material.AIR. This commit reconciles the
differences, so that custom air blocks will return return true from Bukkit
isEmpty() as well.

Test plugin: https://github.com/agaricusb/IncompatiblePlugin/commit/44a2442e98ed31bab1a6d13a38b902c2a7677f16
used with Railcraft's "block.hidden" tracking blocks, automatically added
at player locations periodically. Previously considered non-empty, now correctly
recognized as empty. Plugins can properly support player tracking aura and other
non-vanilla air/empty blocks by calling isEmpty() instead of hardcoding 0 or AIR.