Added multiplayer GUI, fixed bugs

- added a menu to host and join LAN games
- fixed "shaky GUI" glitch
- added support for non-player connections to servers (ex. ping. This is accomplished by making MPClient nonstatic and creating different instances of it for different things)
- server does not download world as soon as client connects, it waits for client to request
- removed background of text chat, fixed text chat overlapping with health bar
- ServerInfo packet sends more info (like player and chunk count)
- added LoginRequest packet to prepare for login authentication
- added entityShootAt and entityGeneratePath packets
- chat messages show player names
- added player disconnect message
- added special parametre to shootAt
- fixed sheep not being rotated correctly