Add support for groups of HttpClientObservers

Currently, we run into the problem where we often bind `HttpClientObserver`s globally for convenience.  It would suck if every {Rating,Message,User,...}Client has to specify that it wishes to pass on authentication, tracking, server info, etc.

This allows you to create configurable groups of observers so that we can bind all the appropriate observers by a common name.

This also removes the "default" httpclient which only causes problems and provides little value.