Enhancement: add operator==, localPort, remoteIP and remotePort to EthernetClient

EthernetServer doesn't provide any way to distinguish clients returned by Server.available() which makes it impossible to implement a Server that handles multiple persistent connections.

I added an operator == to allow this. It checks whether both Clients are assigned with the same socket-register by comparing _sock.

This will not recognize a change in the connection the socket-register handles. So in case the sketch stores a EthernetClient-reference and this clients connection is closed and reconnected by the W5100 you cannot detect this in your sketch by relying on the ==-operator.

To allow detection of this 'client became reconnected'-issue I've added methods localPort, remotePort and remoteIP. I have not includet localPort,remotePort and remoteIP in ==-operator to avoid unnessesary increase in memory-foodprint of the Client-object.

The updated AdvancedChatServer-example demonstrates the use of the ==-operator by distributing incomming messages to all clients but the one the message comes from. (The former ChatServer echos the messages back to the sending client).