Edoc fixes

There are numerous edoc issues in riak-erlang-http-client. <code>rhc.erl</code> has misnamed functions, bad arity, unescaped XML entities and malformed XML. edoc wasn't expanding macros before processing <code>rhc_index.erl</code>. This adds the necessary edoc options in <code>rebar.config</code> so edoc can process <code>rhc_index.erl</code>.