Expose author username in project events API

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Update changelog

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Merge branch 'api-events-username' into 'master'

Expose author username in project events API

Fixes gitlab/gitlabhq#1476

See merge request !1236