Remove opcode from read, async_read (API Change):

fix #446

* Read signatures no longer include `opcode`

* stream::got_binary and stream::got_text inform the caller if
  the current incoming message is binary or text.

Actions Required:

* Remove the `opcode` reference parameter from calls to synchronous
  and asynchronous read functions, replace the logic with calls to
  stream::got_binary and stream::got_text instead.

examples/websocket_example.cpp

@@ -30,8 +30,7 @@ int main()
 
     // Receive WebSocket message, print and close using beast
     beast::multi_buffer b;
-    beast::websocket::opcode op;
-    ws.read(op, b);
+    ws.read(b);
     ws.close(beast::websocket::close_code::normal);
     std::cout << beast::buffers(b.data()) << "\n";
 }