Handle connection events properly to reflect the real connection status

[vinnylinck]

iOS implementation should check stream status before return Open Callback.

[vinnylinck]

Once socket on iOS works with events, the proper implementation basically means:

1. open socket connection
2. returns OPEN operation status (if plugin connected successfully)
3. When receiving the NSStreamOpen event, log that this connection was stablished.
4. The event should change the "connected" flag
5. Other events should be monitored to change the connection flag properly

As a result, isConnected method basically will returns the flag itself, making things simpler and increasing the accuracy of this information.