Add last active and modified dates to tabs

These changes lay the foundation for future work built around tabs.  By having an active and modified date, you can do a few things:

* Sort the `cmd-t` palette by  date.  In Sublime I used this to jump back and forth between two files I'm working on, but Atom sorts this list alphabetically so that dynamic is lost. 
* Automatically close tabs after a certain time of inactivity.  Sometimes I don't do the mundane work of cleaning up previously opened tabs.  If I haven't touched a tab in a few hours, Atom could automatically close it.
* Style tabs based on their age.  Think something like the heat map on [GitHub's blame view](https://github.com/atom/atom/blame/master/README.md). 

/cc @kevinsawicki