[Azoth] Moved /nick to MuCommands.
[Azoth MuC] Added /subject command, closes #1664.
[Azoth] Slightly fixed code style.
[Azoth] Moved conf close handler from ChatTab to Core.
[Azoth] Moved /leave to MuCommands for #1665.
[Azoth MuC] Added /part synonim, closes #1665.