Make Command-{ and Command-} switch active channels.

This is the standard previous/next tab key-combination for many
applications like web browsers, terminal emulators, and other messaging
clients. It would be great if LimeChat also supported it.

This patch has been tested on OS X 10.8.3.
Merge pull request #158 from smerrill/add-curly-braces

Make Command-{ and Command-} switch active channels.