Added new column in notebook list: last_modified.

Continuing on from this PR: https://github.com/ipython/ipython/pull/4596
Tested on the desktop, but not on any mobile or tablet. 

When the notebook name gets too long, the solution here is that text overflow happens. 

![1546-final](https://cloud.githubusercontent.com/assets/954858/3002912/8a5fd180-dd47-11e3-960b-366daa4e355b.png)