Ability to detect, whether join (or other event) was successful or not

[Cysioland]

Scenario:

1. I do Client.join() with callback, supplying incorrect channel key
2. Then I realize, what've I done, and supply correct channel key

   Desired behavior:

3. Callback should be called only once, when we successfully enter the channel

   Actual behavior:

4. Callback gets called twice

   Additional features, while we're at it

5. When join doesn't happen due to ex. user needing invitation or with aforementioned channel key error, there should be the way to bind to it easily, and not just parse the 'error' callback