Add additional constructor for use after OAuth connection

When using the web server flow of the OAuth2 protocol to connect to MailChimp, you don't get an API key of the same form as supplied from the management portal.  In particular, you get two parts: an access token (in OAuth2 terms) which functions as an api key [but crucially doesn't have a data center prefix on the end], and a separate data center prefix.

As a result, we need a further constructor for MailChimpManager which takes these two parts separately.

It would, I suppose, be possible to have combined them in the caller - only to have the library take them apart again, but that seemed wasteful and confusing.