Added the channel browser

The channel browser is accessible via the tool strip and file menu, or
by issuing a /list command to yaircc. Results will be cached to prevent
the need to requery the server constantly, but can be forced by pressing
the refreshing button.
Changed the channel browser to be only accessible once successfully registered on the network.
Merge pull request #14 from intninety/enhancement/channel-browser

Enhancement/channel browser