DONOTMERGE - Guillaume works

Brake compatibilities with actual cozy-calendar and actual cozy-sync, to achieve these :
* changed date convention in database: data and API is in UTC, the client has the Cozy's timezone
* client's dates are now handled with moment-timezone
* added all-day events support
* added event's reminder support


+1, I didn't do it because i was lazy to change it in cozy-sync & cozy-calendar, but if this PR is going to be cozy-ical 1.0.0, we should do it.