Fixed mob spawners being treated as solid blocks for the orebfuscator

Mob spawners are treated as solid blocks as far as the game is concerned for lighting and other tasks but for rendering they can be seen through, therefor we special case them so that the antixray doesn't show the fake blocks around them.
Upstream merge
Add 1.7.6pre1 support.

This doesn't require any configuration changes and will support 1.7.2 / 1.7.5 etc as well.
Pass the offline/bungee uuid to PreLogin events (Closes #147)
Merge remote-tracking branch 'remotes/origin/1.7.6'
Prevent player skulls from crashing 1.7.6 clients. They'll be missing their textures due skin changes
Convert player heads async
Close the inputstreams/connections used for head conversion
Don't spam warnings when Mojang's API goes down. (Which happens a lot)
Check the length of the response as the API can sometimes be incorrect
Fix 1.7.7 support (Bungee will still have issues)
Fix the console issue

QueueLogAppender doesn't like being added twice
Use authlib to look up skull textures.

Might cover a case I missed
Don't convert skulls if the request fails for some reason
Lets say this didn't happen
Find and fix broken player skulls and handle skulls with incorrectly cased player names
Display 'Spigot' in client crashes, server lists and Mojang stats
Validate skins server side to try catch some out of date plugins
Display 1.7.8 as the current version
Remove the server skin validation

Doesn't really help in catching the plugins causing the issues