Buffer: send and receive binary data over Channel

(also some minor fixes in channel)

Examples/Channel.py

@@ -243,11 +243,6 @@ def client(destination_hexhash, configpath):
     # And create a link
     link = RNS.Link(server_destination)
 
-    # We set a callback that will get executed
-    # every time a packet is received over the
-    # link
-    link.set_packet_callback(client_message_received)
-
     # We'll also set up functions to inform the
     # user when the link is established or closed
     link.set_link_established_callback(link_established)
@@ -330,7 +325,7 @@ def link_closed(link):
     time.sleep(1.5)
     os._exit(0)
 
-# When a packet is received over the link, we
+# When a packet is received over the channel, we
 # simply print out the data.
 def client_message_received(message):
     if isinstance(message, StringMessage):
@@ -348,7 +343,7 @@ def client_message_received(message):
 # starts up the desired program mode.
 if __name__ == "__main__":
     try:
-        parser = argparse.ArgumentParser(description="Simple link example")
+        parser = argparse.ArgumentParser(description="Simple channel example")
 
         parser.add_argument(
             "-s",