Add css class for status bar (command, insert, visual)

![2014-06-05 14_02_17](https://cloud.githubusercontent.com/assets/557961/3183587/061b08ea-ec6f-11e3-87f2-db88cf8652d3.gif)

It can change vim mode looks on status bar.

thx.
We should be able to do `$('#status-bar-vim-mode').removeClass('status-bar-vim-mode-insert status-bar-vim-mode-command status-bar-vim-mode-visual')` and clean this up slightly