Fixed NullPointerException in yCoordHelper

I believe I've fixed the cause of a NullPointerException that some
players have been experiencing. It seems to have been caused by
unregistered block IDs being present in Limbo. Possibly because people
removed mods and they had generated structures in Limbo. Or the players
had placed blocks and then removed the mods, or changed block IDs
around.

This fixes issue #47.
Fixed Export Height Bug

Fixed a bug in DungeonHelper that could cause dungeons to get clipped
during export if they extend past Y = 127. We used
world.getActualHeight() to obtain the height of the pocket, which should
be 256, but getActualHeight() returns 128 for dimensions with no sky.
Pocket dimensions are set to have no sky, so this becomes an issue.
Minor Change

Added a note on a strange condition in EventHookContainer.
Removed Limbo Decay

Removed the code associated with Limbo decay from EventHookContainer,
LimboBlock, and CommonTickHandler. DimHelper still has blocksToDecay
since removing it could cause deserialization failures. However, nothing
should actually use that list. I also removed several unused or
redundant methods (e.g. overriding methods and putting in exactly the
same code as in the super class), fixed indentation and renamed cryptic
variables. We should have a rule against allowing cryptic variable names
like "par2", even if they're inherited from Forge. I'll implement proper
limbo decay in the next commit.
Reorganized Code in CommonTickHandler

Reorganized code in CommonTickHandler. Should be a little more readable
now. This is in preparation for reimplementing Limbo decay.
Reimplemented Limbo Decay

Reimplemented Limbo Decay in two forms. Firstly, Unraveled Fabric
accepts random ticks now and has a 50% chance of searching the 6 blocks
against its faces for blocks to decay. The average time for this decay
to occur is about 2 minutes and 17 seconds. The decay progresses a
little slowly because of having to go through stages. We might want to
consider decaying straight into Unraveled Fabric, or at least having
fewer stages. This approach is better than randomly decaying isolated
blocks because it looks like decay is spreading from the Unraveled
Fabric.

Secondly, every tick, we pick a random block from each active section in
Limbo and turn it into Unraveled Fabric immediately. Note that a section
is a 16x16x16 block cube inside a chunk. This is "fast decay", and it's
meant to stop players from avoiding Limbo decay by building floating
structures. It's not immediately obvious if you place a single block,
but if you build a 5x5 block platform, the average time for some block
to get converted drops to about 8 seconds, and it spreads from there.

Most of the logic for this is in a new class: LimboDecay. It's worth
studying whether this new implementation affects Minecraft's
performance. I'm not completely sure whether that would be an issue. The
frequency of either decay type can be decreased to improve performance.
Minor Change

Minor change. Changes comments in LimboDecay slightly so that they act
as Javadoc comments instead. That was my original intention.
Reduced Frequency of Fast Limbo Decay Operations

Added an interval to CommonTickHandler so that fast decay operations are
performed less frequency. The previous rate was unnecessarily fast. This
will also reduce the performance impact of fast decay considerably,
although testers did not report any noticeable performance decreases.
Merge pull request #52 from SenseiKiwi/1.4.1fixes

Fixed NPE in yCoordHelper, Other Bugs, Reimplemented Limbo Decay