Create client method for [PUT] /activity API call

Signed-off-by: Balthazar Rouberol <balthazar.rouberol@mapado.com>
Fix typo

Signed-off-by: Balthazar Rouberol <balthazar.rouberol@mapado.com>
Merge pull request #4 from mapado/create-activity

Create client method for [PUT] /activity API call