Normalise screen size notation

Hi There!

I have taken the idea in this issue and done the following 
https://github.com/twitter/bootstrap/issues/7706

- Changed the less file - responsive-utilities.less
- Recompiled bootstrap-responsive.css and place in docs\assets\css
- Changed the notation in the docs html files wherever it was called
- Changed the notation in the Moustache templates 

----
.visible-phone -> .visible-sm
.visible-tablet -> .visible-md (md for "medium")
.visible-desktop -> .visible-lg
[similarly for .hidden-*]