Remove HTTP methods from RightScale class.

Reserve the namespace in the RightScale class for higher-level concepts
that are meaningful to RS consumers (e.g. list_instances()).

This change also simplifies using rightscale.RightScale objects such
that callers do not need to explicitly login.  Instead they can get down
to business:

    rs = RightScale(account_id, refresh_token)
    rs.list_instances()