Help system

First implementation for #343 : 

* help for home page only (other pages could be implemented later)
* help is launched with a new item in the top menu 
* the help tour is not automatically started on first login (user experience was too clumsy). Maybe we could simply mention its existence in the welcome tour.

I've tried to keep it very simple so that it could be shipped with 2.0 (which would be great !), but if you feel it's too risky we can push it to 2.1.