Added retrying on Rift Gateway generation

Added a loop that allows RiftGenerator to retry generating a gateway if
it picks an invalid location for it. It has a limited number of tries
before it gives up and doesn't generate a gateway. I haven't added the
logic for avoiding generating on top of trees, but that'll be easy to
add now.
Fixed Gen Directionality Bug and Minor Tweak

I fixed a subtle bug in RiftGenerator. The code for picking random
coordinates on the surface of a chunk would subtract a random amount
from the coordinate of the chunk's corner. Unfortunately, subtracting
meant that we would almost always generate coordinates OUTSIDE the
intended chunk. The correct calculation was to add, not subtract. This
meant that if you walked in a direction in which our gateways were
generated outside of existing chunks, then their data would be lost and
no gateways would generate!

To reproduce this bug, max out the chance of generating gateways and fly
in the direction that decreases X and Z. You must fly into new chunks.
After passing the gateways produced when you spawn (if this is a new
world) you'll see that gateways don't generate on the landscape.
However, if you change directions, they'll begin generating again.

In addition to fixing that bug, I also tweaked all the block setting
calls in RiftGenerator so that they'll trigger block updates and send
update packets to the client. I think triggering block updates will
prevent unusual problems with floating sand, gravel, etc.
Improved Gateway and Rift Generation

Added checks for the material that a gateway is being built upon. Now
gateways no longer generate on top of trees. Combined with multiple
generation attempts, there is a good chance that they will find a gap
between trees to generate at ground level. I also added checks to stop
gateways and rifts from generating on top of the Nether's bedrock.
Cleaned up ItemStableFabric

Cleaned up the indentation of ItemStableFabric and corrected some faulty
String comparisons.
Fixed Dungeon Listing and Formatting Bugs

Added a workaround so that the dungeon list produced by
CommandAddDungeonRift does not have repeated names. Also cleaned up the
code a bit - it's a mess in there. Part of the rewrite eliminated the
bug that caused some dungeon names to be preceded by a backslash (or
slash). I've noticed a different bug now - the dungeon tutorial file is
being included in the list of dungeons. I'll be fixing that in the next
commit.
Fixed Tutorial Listing Bug and Improved Listing

Fixed the bug that caused the dungeon creation tutorial to be listed as
a schematic. We were listing all files in the custom schematic directory
as a schematic, even though that file wasn't. I added filtering so that
we only look at files that end with ".schematic". Also improved dungeon
listing by sorting the name list in alphabetical order. That makes the
list much easier to read through.
Added a comment

Just added a comment in DungeonHelper.
Merge pull request #26 from SenseiKiwi/master

Improved Rift Gateway Generation and Fixed Several Bugs