doc: move admin_api.md to other_apis.md
api: remove admin prefix from members API
doc: remove trailing slashes
Merge pull request #1481 from bcwaldon/no-more-admin-api

Remove "admin" from /v2/admin/members
Merge pull request #1493 from bcwaldon/etcdctl-members

etcdctl members [list|add|remove]
client: pass around statuscode instead of Response

There's no real need for do and doWithTimeout to return Responses when
the only field of interest is the status code.

This also removes the superfluous httpMembersAPIResponse struct.