Change EraseLogin to return IObservable when done

We shouldn't have void methods. We may want to ensure this is complete
before we do the next thing.
Change InvalidateAllObjects to return IObservable

Same as before, we want to be able to see when this finishes.
Change other void methods to return IObservable

NO MORE VOID!
Changed IObjectCache interface to not return void
Merge pull request #37 from github/erase-login-improvement

Erase login improvement