Trainings data as YAML, filtered to display only future ones

Caveats :
- despite what being said on [official documentation](http://jekyllrb.com/docs/datafiles/), Jekyll can't load JSON data. I wanted to reuse `trainings.json` from www, but that's not possible.
- even if only future trainings are displayed, that's only true at generate time (i.e. git push time), not at HTTP request time : if a training is in two days when blog is deployed, it will still be displayed in three days.
- there is still work to have French localized dates, but [that's possible](http://alanwsmith.com/jekyll-liquid-date-formatting-examples)