Add tabs:toggle command for toggling the Tab Bar

I've added the ability to toggle the visibility of the Tab Bar with cmd-ctrl-t by default, or the 'tabs: toggle' command.  I used toggleClass('visible') so that it could be animated easily via css, but left it as a simple toggle as a base. 

![toggle-tabs](https://f.cloud.github.com/assets/230391/2338043/d1d8ba12-a4a4-11e3-9c1a-9d8fd71fd597.gif)