juju/api/usermanager: add UserInfo

UserInfo was recently added to the server API. This PR adds the client side UserInfo method.
There's a lot of unnecessary bits here. There's no need to define the function type. Just do this:

```go
var Call = func(st *api.State, method string, params, result interface{}) error {
    return st.Call("UserManager", "", method, params, result)
}
```

Also, should Call really be exported? It's very much an internal detail. Is it possible to leave it unexported and use a export_test.go to allow access to it for mocking for tests?