Add a dedicated field for joining channels.

For many of our users, we're guiding them through their first experience with IRC. Most GUI clients have a dedicated field for joining a channel. This PR adds a join field below the channel list. It supports channel keys just by entering '#channel key123'.
![Subway 2013-03-29 18-21-15](https://f.cloud.github.com/assets/255023/319422/f964bfa6-98be-11e2-9313-345cb62e848b.jpg)