Fix NPE when loading second single-player world

After leaving a single-player world, getServer() can return null, resulting
in the stack trace below, "Fatal errors were detected during the
transition from SERVER_STOPPING to SERVER_ABOUT_TO_START",
"The ForgeModLoader state engine has become corrupted" and the client
getting stuck forever.

Exception in thread "Server thread" java.lang.NullPointerException
at cpw.mods.fml.client.FMLClientHandler.serverStopped(FMLClientHandler.java:620)
at cpw.mods.fml.common.FMLCommonHandler.handleServerStopped(FMLCommonHandler.java:468)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:471)
at net.minecraft.server.ThreadMinecraftServer.run(SourceFile:583)
Merge pull request #284 from smcv/uninit

Fix NPE when loading second single-player world
Fix injection of tweaker into system
Herpaderp. Make addURL accessible. *sigh*
And use the parent that's actually going to be valid. *sigh*
Update to new installer that uses xz compression and better support for non-standard jvms
Add debugging for deobfuscation

Add negative caching for field and method maps. Should shortcut some work

Fix handling the case where a class doesn't exist, but still needs deobf

Ask the patch manager for the bytes for the class, not the classLoader

Fix up some more places where we should look for a patched class

Fix problem with empty source classes and runtime deobfuscation
Use the negative cache in latest legacylauncher.
Update for launchwrapper 1.8
Separate deobf tweaker so it runs last. Should fix problems with cascaded tweakers
expecting an obf environment.
TYPOS!!!
Fix deobf tweaker
Add to both the classloader and the parent
Cross-modsystem compatibility fix with thanks to Mumfrey @ liteloader
Attempt at a shared modlist implementation - should allow visibility between liteloader and fml for "mod list display"
Fix md5s for new checksums.sha1 in scala libraries.
Fix null killing the server
Add in support for Optional interfaces and methods. Be gone coremods!
Optional shouldn't be constructable itself. It's purely a wrapper thing.
ModLoader is officially deprecated. It will all cease to be with 1.7.