Decrease pressure on garbage collector

[zbindenren]

Hi

I observed that on each api call a http.Client is created: https://github.com/scottdware/go-bigip/blob/master/bigip.go#L119

I would like to refactor that code to initiate a http.Client in NewSession and use this client for each call.

To do that, the Transport has also to be created in NewSession. My plan:

• move the Transport configuration to ConfigOptions like

type ConfigOptions struct {
	APICallTimeout time.Duration
        Transport *http.Transport
}

• do not use InsecureSkipVerify: true, like here

What do you think about that?