Introduction of IXWebSocketSendData (#347)

* Introduction of IXWebSocketSendData that makes it possible to not only send std::string but also std::vector<char/uint8_t> and char* without copying them to a std::string first.

Add a sendUtf8Text() method that doesn't check for invalid UTF-8 characters. The caller must guarantee that the string only contains valid UTF-8 characters.

* Updated usage.md: sendUtf8Text() and IXWebSocketSendData

CMakeLists.txt

@@ -110,6 +110,7 @@ set( IXWEBSOCKET_HEADERS
     ixwebsocket/IXWebSocketPerMessageDeflateCodec.h
     ixwebsocket/IXWebSocketPerMessageDeflateOptions.h
     ixwebsocket/IXWebSocketProxyServer.h
+    ixwebsocket/IXWebSocketSendData.h
     ixwebsocket/IXWebSocketSendInfo.h
     ixwebsocket/IXWebSocketServer.h
     ixwebsocket/IXWebSocketTransport.h