Fix issue #19

both MEF and Nancy execute the `Admin` plugin's .ctor. so `_listeners` contains duplicate entry for `Admin` plugin.  
i still need this for two reason:
*  ` if (_listeners.Contains(listener))` using the default equality comparer as defined by the object's implementation of the IEquatable<T>.Equals. and in our case this always produce `false`
* i can use `Action<Response> responseAction` for comparer, but maybe one message could mapping to more than one action.(one < - > many)

last, let the Listener be IEquatable may be a little werid. or you have better ideas plz let me know:)