Add createUser(), removeUser() and authWithPassword()

I added createUser(), removeUser() and authWithPassword().

As a consequence of that, I also needed to change AuthResponse.auth to type JsObject rather than String, although perhaps it should just be untyped.

Thoughts?

Also, if I execute async methods within a Dart test, there seems to be no way to wait for completion other than chaining async calls together using .then().  Which is what I do in the auth-credentials test group.  The test seems a bit monolithic, but if I write separate tests for createUser(), removeUser() and authWithPassword(), then there's a race toward failure.

How do you want to see these tests structured?  I would be happy to fix.