Created "Server::receive()"

Pushed socket-specific stuff there.
Removed "Server::run()"

Instead of having a function for it, push it all to the main() function
Abstracted away things to "UDPServer"

It inherits from "UDPSocket", making easy to receive data.
Abstracted away things to "UDPClient"

Now you can easily create UDP clients and servers on your applications.

This one is quite simple, since it merely echoes from one side to
another, but we could easily make a two-way conversation.
Removed "Client::run()"

Now the "main()" has all the logic of the application.
Put all the receiving code from "Server" to "UDPSocket"
Removed unnecessary #includes from "Client" and "Server"