Add Alternative to inject Add_this service into template

I know we killed mans for less than that, so if this is an other bad idea, sorry :)

The problem I see with html_social function:
- it seems to only be used for Add_this at the moment
- if you want to change the add_this code you have to rewrite the full template base_helper.html
- if you change the template you lose your add_this code
- not really neat if you want to keep the add_this code up to date

So I wonder if it won't make more sense to take the same approach used for the google analytic, and make this code live in the conf.py.

Here a pull-request to illustrate, if it's worth talking about it, I will continue on the patch.