EthernetServer - added begin(port) and ctor without parameters

Author: JAndrassy

docs/api.md

@@ -884,11 +884,12 @@ Tells the server to begin listening for incoming connections.
 
 ```
 server.begin()
+server.begin(port)
 
 ```
 
 #### Parameters
-None
+- port (optional): the port to listen on (int)
 
 #### Returns
 None

src/Ethernet.h

@@ -255,10 +255,12 @@ class EthernetServer : public Server {
 private:
 	uint16_t _port;
 public:
+	EthernetServer() : _port(80) { }
 	EthernetServer(uint16_t port) : _port(port) { }
 	EthernetClient available();
 	EthernetClient accept();
 	virtual void begin();
+	void begin(uint16_t port) {_port = port; begin();}
 	virtual size_t write(uint8_t);
 	virtual size_t write(const uint8_t *buf, size_t size);
 	virtual operator bool();