Allow channel tabs to be sortable #79

Hey guys, my first contribution here with a feature that I missed when I started using komanda this week.
This simply add a native drag'n'drop functionality to the channel tabs in the sidebar, allowing them to be sortable.
If a core member can review this, I would be thankful.
Fix #79 

Feature demo here:
![screen recording 2014-12-18 at 08 07 am](https://cloud.githubusercontent.com/assets/800417/5486459/fd0f268c-868d-11e4-8b6e-5a876d324b27.gif)