Added a factory method that supports BASIC auth.

[kohsuke]

My original requirement is to be able to send an HTTP basic authentication along with requests.

I took the liberty to also add a version of a method that takes arbitrary RequestInterceptors to support other possible modes of authentication, and general control. I think it's worthwhile doing this now to avoid ending up with 20 different overloaded flavors of the getInstance() method, but I can imagine you might not want to expose Feign or perhaps go the other way and not have the getInstanceWithBasicAuth method.