Avoid sending a signal or killing the session if the process is not alive

[nodo]

Fix issue #258

[williammartin]

Looks sensible to me but I'm wondering if we can consolidate on using signal in Kill like:

func (s *Session) Kill() *Session {
	return s.Signal(syscall.SIGKILL)
}

Just seems we could get rid of an extra conditional at the cost of a slightly less lower abstraction, which I think on the whole is a win. WDYT?

[nodo]

I think it makes sense @williammartin.

I can also see that using Signal in kill is consistent with Terminate and Interrupt (https://github.com/onsi/gomega/blob/master/gexec/session.go#L161-L181). Even though they don't have a correspondent method in process (which is the case for kill), the code would like nicer.

[williammartin]

LGTM thanks