footer with discreet IPython version

Cause it is true that it is difficult by just staring at the screen to guess IPython version, will be even more as the UI stabilize.
Asking user to type things is kinda difficult sometime.

"Scroll down and look at the bottom-right corner of the screen, do you have 3.0 version or above" is easier. 

![capture d'écran 2014-09-26 a 15 42 56](https://cloud.githubusercontent.com/assets/335567/4421307/c6b1336a-4582-11e4-92ba-aee6aab2e7dd.png)

I'm wondering should we have `IPython-Server vX-Y-Z` less confusing than using IPython for everything.
Should I include commit-ish for dev ? 
Seriously considering a more informative `About` dialog.

Yes, the implementation is wonky, cause of that #$%^ pager that imply that not the same div scrolls depending on  pages. 
Cached sys_info in an intermediate variable. I'll leave it as accessible from every template if we want an about page.