Added SetReuseAddress() method to WebSocketServer

By using `SetReuseAddress()` before `Start()` you don't have to wait for TIME_WAIT to timeout. Normally you will recieve the following exception if you try to start websocket-sharp on the same port within TIME_WAIT:

    [SocketException (0x2740): Only one usage of each socket address (protocol/network address/port) is normally permitted]