improve state handling

the running/stopped state handling is incorrect. Since func (c *Client) Start() (bool, error) is blocking and only left on an error or after func (c *Client) Stop() (bool, error) has been called, the boolean c.started is set too late to true (l.76 in Start()) - when actually the client is about to stop.

Instead I would like suggest to implement sth like:

func (c *Client) IsStarted() bool {
	return c.server != nil
}