Add nick coalescence for multiple lines.

This isn't exactly inline with the original way whisper worked, and if you reject it because of that, I understand.

Basically, when a single user sends multiple lines, this hides their nick (and the divider lines) from successive messages.

![screen shot 2014-10-23 at 3 33 02 pm](https://cloud.githubusercontent.com/assets/168193/4762641/be428796-5b04-11e4-857e-b1395513e8d7.png)

This is how hipchat and several other "modern" chat applications work, and it leads to a less busy UI.