Fixing some typos + changing "Calendrier" for "Agenda"

The appropriate translation of "calendar" for this service is "agenda".
See e.g. http://transvision.mozfr.org/?recherche=calendar&repo=central&sourcelocale=en-US&locale=fr&search_type=strings
Merge pull request #169 from goofy-bz/patch-3

Fixing some typos + changing "Calendrier" for "Agenda"
Add no description placeholder on events without descriptions. Fix #123 .
Merge branch 'master' of github.com:jacquarg/cozy-calendar