Tab reorder

In Chrome Linux and the Gnome environment (at least Gnome 3), `ctrl-shift-pagedown` is a shortcut to reorder tabs.
Here is the Chrome example:
![chrome-tabs](https://cloud.githubusercontent.com/assets/6003742/4966247/a300e9cc-67b7-11e4-96df-7b6f3cfdd8ed.gif)

Anytime I get back to Atom I tried to reorder some tabs just like with Chrome, but it was missing this shortcut. I thought it would be nice to have it. Here is how it looks:
![atom](https://cloud.githubusercontent.com/assets/6003742/4966259/67bf84a8-67b8-11e4-91c4-c21e056d2fd9.gif)

You may have noticed that Chrome has a nice animation, which I didn't try to implement, but if you would like to have it then let me know what are your thoughts on that so maybe I can start working on it.

Thank you