Client improvements: initial disconnect, futures for methods, error stream

[FZambia]

Next iteration of improvements here:

1. Return Futures from connect, disconnect, subscribe, unsubscribe - addresses Make all async functions return a Future #31
2. On initial connect fire DisconnectEvent on connection error - this makes behavior of centrifuge-dart similar to all other our clients - addresses Handle connection errors #56
3. Add client error stream to consume ErrorEvent - each transport failure will emit error to this stream - addresses Handle connection errors #56
4. Refactor subscription statuses - add subscribing and error statuses
5. Do not call UnsubscribeEvent if subscription not successfully subscribed (in subscribed state).
6. Update disconnect reasons due to failed connection and calling Client.Disconnect method - make it more similar to all other connector libraries in ecosystem
7. Improve/fix examples
8. Add default transport timeout (10 sec) – on connect and subscribe timeouts client will auto reconnect, calls like publish, history, rpc can now throw TimeoutException. Also - properly pass timeout to the transport (was not before!). Again – this makes client behave similarly to all other connectors.
9. Add presence and presenceStats methods for Subscription and on client top level (for server-side subs).
10. Support streamPosition in SubscribeSuccessEvent
11. Support streamPosition in ServerSubscribeEvent
12. Support data in ServerSubscribeEvent
13. Implement send method to send async messages to a server
14. Fix deletion during iteration over map when working with server-side subscriptions
15. Better event String representations
16. Trigger unsubscribe event when unsubscribe called in disconnected state.