[API/Pods] Add endpoint to remove an owner from a pod

Needed for https://github.com/CocoaPods/cocoapods-trunk/issues/35

\c @kylef @floere @alloy 
@segiddins No, that’s about when the user is not authenticated at all. I.e. it tests the `:requires_owner => true` option. The test I’d like to see is _with_ an authenticated user who does **not** own the pod and thus may not delete owners.