EXC_BAD_ACCESS (code=1) on disconnect from server

Firstly, thanks for a great library, along with Faye, I've found it to be really useful!

For various reasons, my application tends to recycle it's Faye client from time-to-time (connecting to different URLS, reauthenticating, disconnecting the client when the connection is not required, etc). I frequently experience problems with dangling non-safe weak references shortly after disconnecting the Faye connecting, especially if I release my FayeClient before the server has responded to the disconnect message.

The problem occurs in objc_retain, when trying to perform `webSocket:didReceiveMessage` in `SRWebSocket's_handleMessage`

```
- (void)_handleMessage:(id)message
{
    SRFastLog(@"Received message");
    [self _performDelegateBlock:^{
        [self.delegate webSocket:self didReceiveMessage:message];
    }];
}
```

The reason it's happening is that the delegate is not being nulled out before the delegate object is disposed. This patch seems to fix the problem for me.