Public api activity

Open:
- [x] get_navigation
- [x] clarification on active parameter within get_navigation
- [x] get_filter https://github.com/owncloud/activity/blob/issue/82/lib/data.php#L235
- [x] get_filter https://github.com/owncloud/activity/blob/issue/82/lib/data.php#L304

requires https://github.com/owncloud/core/pull/9412
Just for the record, this needs fixing before merging.