avoid API call to GetList when calling CreateMember

[alexflint]

In the GetList function there is an API call to mailchimp that gets information about the mailing list. Is it really necessary to make this API call before adding an email to a list with CreateMember? It adds some latency to my code, and also the GetList API call sometimes times out on the mailchimp end (this is not the fault of this library of course, it would just be nice not to need that API call to always succeed)