SetTimeout() may do nothing?

To reproduce:
```go
t, err := telnet.Dial(...)
t.SetTimeout(time.Duration(100) * time.Millisecond)
```
After this examine t and timeout has old default value = 5s. The telnet.DialTimeout(...) work propertly.
May be fix:
```go
// SetTimeout changes default or start timeout for all interactions
func (t *Telnet) SetTimeout(timeout time.Duration) {
	t.timeout = timeout
}
```

