disable send button while sending message

After being spammed by some colleagues that didn't notice that their message was being sent I added this little UX tweak, it disables the send button and updates it's title to `Sending..`:

![img_1184](https://cloud.githubusercontent.com/assets/749864/3675428/b87e2848-127e-11e4-9bc6-a94ee24397bc.PNG)