"Mute sounds" menu item

![Screenshot](http://i.imgur.com/TILjVgg.png)

**What?**
I basically just added a menu item that allows for users to quickly mute Limechat's sound notifications.

**Why?**
Sometimes I want to know what everyone is saying in a channel and other times I want everyone to shut the f*** up already! This seemed to be a nice in-between feature :smiley: 

**Notes**
This is my first pull request for an Objective-C project, so I apologize in advance if my code is a bit rusty. I tried to stick with the coding style of the project, but if I messed anything up let me know and I'll get it fixed. Limechat seems to have a really well formatted codebase, so I don't want to be the one to ruin that!

I also think I changed a bit too much in `MainMenu.nib` (version control with these files is terrible, ugh), so let me know what needs to be changed there.

Feedback would be super appreciated and I hope you guys like this!