Support WebDAV group/member API

[eblondel]

https://doc.owncloud.com/server/developer_manual/webdav_api/groups.html

• [ ] Custom Groups
  • [ ] List Groups
  • [ ] Rename Custom Group
  • [ ] Delete Group
  • [ ] Create Group
• [ ] Group Membership
  • [ ] List Members
  • [ ] Add Member
  • [ ] Remove Member
  • [ ] Change Admin Role of a Member
  • [ ] List Group Memberships of a Given User