Stomp::Client's connection's host params should be exposed in the client.

A Stomp::Client has attribute accessors for various host params (specifically, host, port, login, passcode, ssl), but in the current version, those attributes are always going to reflect the first host in the params hash (if the client was initialized from a params hash).

This change gets rid of those instance variables, and makes the corresponding methods delegate to the connection.  Thus, if one host goes down and we failover to another, the client's :host, :port, etc, methods reflect the current state, instead of being unchanged.