Make Xrc::Client ctor to accept hosts parameter

I want to specify server hosts explicitly on connection.  This patch makes it possible.

Reason:
Under some circumstance, XMPP server cannot determined by DNS for domain of JIDs.

E.g. HipChat Server (on-premises edition)
JID = XXX@chat.btf.hipchat.com
Connect host = user.installed.server.other.than.hipchat.com.example.com

I just failed to notice `private` declaration. :sweat_smile:
So I recognized that `hosts` (also `domain` and `port`) were read-only properties by mistake.

Yes, instance variable `@hosts` is just garbage.
May I correct it in the pull-req?