Improved command blocks

* Their command and previous output are displayed on the client
* They have a BlockHandler implementation, so you can't place blocks on
them anymore
+ As a side effect, implemented UpdateBlockEntity
ProtoProxy: UpdateBlockEntity NBT is written
Implemented MC|Brand response
Changed SendBlockEntity format slightly

* Writing NBT is now in Protocol, not BlockEntity files
* Fixed a last output bug
Removed unneeded paramters
Merge pull request #565 from mc-server/commandblockfixes

Command block fixes