Merge pull request #4 from perki/master

Fixed some typos and added `time` (replacing `duration`)
Event types v0.2.1: fixed JSON schema "maxLength" + case fixes
One more JSON schema fix
Event types v0.2.2: fixed "message/twitter" schema
Event types v0.2.3: adjusted "message/twitter" schema again
Event types v0.2.4: minor fixes
added basic sets
Corrected typo in time/year extra key
Event types v0.2.5: replaced "activity/pryv" with "activity/plain"
Merge branch 'master' of github.com:pryv/data-types

* 'master' of github.com:pryv/data-types:
  Corrected typo in time/year extra key
  added basic sets
added count/generic to extra, count/step to definitions