Fix lightened nibble array getChunkSnapshot on extended block IDs

Fix out-of-bounds error in chunks with extended (4096) block IDs:

13:34:12 [SEVERE] [dynmap] Exception while fetching chunks:
java.lang.ArrayIndexOutOfBoundsException: 4096
    at org.bukkit.craftbukkit.v1_4_R1.CraftChunk.getChunkSnapshot(CraftChunk.java:1

http://pastie.org/6368930

Thanks to mikeprimm for this fix.

Original MCPC+ commit:
MinecraftPortCentral/MCPC-Plus@790276739766cb1733cc0787aaeb88ae6a507608
Merge pull request #27 from agaricusb/fix-nibblearray4096

Fix lightened nibble array getChunkSnapshot on extended block IDs