* Move `inviteUsersToRoom` to `RoomUpgrade`
This method is only used in one place, uses only public methods, and is
undocumented. Let's move it to the place where it is used, to simplify the API
for `RoomInvite`.
* Simplify `inviteUsersToRoom`
`inviteMultipleToRoom` basically never throws, so this code was effectively
unreachable.
* Update MultiInviter to take an options object
I'm going to add another option, so an options object is going to be more
flexible.
* Jump through the coverage hoop with another test
