Adds button for joining a channel without writing /join

Fixes issue #123 and adds a button under the channel list for easily inputing a
channel name and pressing enter. This should make it more easy for new users to
IRC, joining channels. Someone with a better feel for UI may want to look at
how to make this prettier and even simpler.

It looks like this:

![screen shot 2014-02-17 at 15 30 21](https://f.cloud.github.com/assets/116978/2186120/03565ff2-97e1-11e3-8e54-625cfe45d75e.png)
![screen shot 2014-02-17 at 15 29 59](https://f.cloud.github.com/assets/116978/2186121/036f7410-97e1-11e3-9532-e2bc93e8eb82.png)

One may perhaps look into how HipChat does it, which looks like this:

![screen shot 2014-02-17 at 15 38 18](https://f.cloud.github.com/assets/116978/2186134/2a048e12-97e1-11e3-8d54-5b7b672ee717.png)